The 10 firefighters who received the call shortly before 6 p.m. - a big fire at the nearby port of Beirut - could not know what awaited them.

The brigade of nine men and one woman could not know about the stockpile of ammonium nitrate warehoused since 2013 along a busy motorway, in the heart of a densely populated residential area - a danger that had only grown with every passing year.

They and nearly all the population of Beirut were simply unaware. They were not privy to the warnings authorities had received, again and again, and ignored: ammonium nitrate is highly explosive, used in fertilizer and sometimes to build bombs. The stockpile was degrading; something must be done.

They knew, of course, that they lived in a dysfunctional country, its government rife with corruption, factionalism and negligence that caused so much pain and heartbreak. But they could not know that it would lead to the worst single-day catastrophe in Lebanon's tragic history.

Across the city, residents who noticed the grey smoke billowing over the facility Aug. 4 were drawn to streets, balconies and windows, watching curiously as the fire grew larger. Phones were pulled out of pockets and pointed toward the flames.

The firefighters piled into a fire engine and an ambulance and raced to the scene - and to their doom.

Seven years ago, a ship named the Rhosus set out from the Georgian Black Sea port of Batumi carrying 2,755.5 tons of ammonium nitrate destined for an explosives company in Mozambique.

It made an unscheduled detour, stopping in Beirut on Nov. 19, 2013. The ship's Russian owner said he struggled with debts and hoped to earn extra cash by taking on pieces of heavy machinery in Lebanon. That additional cargo proved too heavy for the Rhosus and the crew refused to take it on.

The Rhosus was soon impounded by Lebanese authorities for failing to pay port fees. It never left the port; it sank there in February 2018, according to Lebanese official documents.

The Port of Beirut is considered one of the most corrupt institutions in a country where nearly every public institution is riddled with corruption. Port officials are notorious for taking bribes. A bribe from an importer, for example, will ensure an incoming shipment is mislabeled to get lower customs duties - or escapes duties and taxes completely. Confiscated goods are sometimes sold off on the sly for a profit.

For years, Lebanon's ruling political factions have divvied up positions at the port and handed them out to supporters - as they have ministries, public companies and other facilities nationwide.

The longtime head of customs is known to be a loyalist of President Michel Aoun, for example, while the head of the port is in the camp of Saad Hariri, the Sunni leader who has repeatedly served as prime minister. The Hezbollah militant group and, even more, its Shiite ally the Amal faction headed by Parliament Speaker Nabih Berri, also have loyalists at the port, though Hezbollah doesn't have the same influence as it does at, for example, the airport, which it controls and uses to ferry in cash from Iran.

The result is a port divided into factional fiefdoms that don't necessarily work together and are sometimes outright rivals. Individual port authorities are sometimes more concerned with their scams than with proper functioning. And government officials avoid looking too closely at goings-on at the port to protect their loyalists.

The first known warning came on Feb. 21, 2014, three months after the ship docked at the port.

In a letter to the customs authority's anti-smuggling department, senior customs official Col. Joseph Skaff wrote that the material on board was "extremely dangerous and endangers public safety."

It is not known if Skaff ever received a response or if he sent other letters. He was found dead outside his house near Beirut under mysterious circumstances, shortly after he retired in March 2017. At least one medical report suggested he might have been murdered.

Skaff's son, Michel, said he was killed by a blow to the head. He said his father dealt with other sensitive matters, including drug trafficking. "Someone maybe was trying to hide what is happening at the port," he said by telephone from his home in New York City.

In the years that followed, Skaff's letter was followed by other correspondence that went back and forth between top customs and port officials and members of the judiciary and the army.

On June 27, 2014, with the ammonium nitrate still aboard the Rhosus, Jad Maalouf, a judge for urgent matters, warned the Ministry of Public Works and Transportation in correspondence that the ship was carrying dangerous material and could sink. He said the ministry should deal with the ship, remove the ammonium nitrate and "place it in a suitable place that it (the ministry) chooses, and it should be under its protection."

It is not clear if there was ever a reply. Ministry officials did not respond to requests from The Associated Press asking for comment.

In October 2014, the ammonium nitrate was moved into the port's Warehouse 12, which holds impounded materials.

A chemical forensic expert, commissioned by the courts and the owners of the ammonium nitrate, got a look at the stockpile soon after. It was "in terrible shape," she said in her February 2015 report. Most of the sacks - she estimated more than 1,900 of the 2,750 sacks- were torn open, their contents spilling out. Some of the crystals had darkened, a sign of decomposition. The sacks were piled so haphazardly that she could not count them to be sure all were still there.

The inspector recommended the chemicals be disposed of according to environmental guidelines. Her report was uncovered by Riad Kobaissi, an investigative reporter with Al Jadeed TV who has followed corruption at the port and within the customs authorities since 2012.

On Oct. 26, 2015, the army command asked customs to sample the material and check the level of nitrogen "and based on that we can give a suggestion regarding them."

The then-head of the customs department, Shafeeq Merhi, wrote back in February 2016, saying an expert found the nitrogen level was 34.7%, a very high and dangerous level, well above the acceptable concentration of around 11%.

The army command responded the following April, saying it didn't need the ammonium nitrate. It asked customs to contact Lebanese Explosives Co. - a maker of explosives for construction of roads and tunnels and for imploding structures - to see if that private company could use it.

If not, the material should be exported at the expense of the ship owner who brought it to Lebanon, the army said in its letter.

An administrator at Lebanese Explosives told the AP that it was "not interested in buying confiscated material because we did not know where they were brought from, what is the quality nor its expiry."

Merhi and his successor as customs chief, Badri Daher, sent multiple letters in the following years to the Courts of Urgent Matters, warning of the danger and seeking permission to sell the material or a ruling on another way to get rid of it.

Daher told the AP and other media that he never received any reply from the court. But Kobaissi, the investigative reporter, found documents showing the court responded each time that it didn't have jurisdiction and that the Public Works Ministry had to decide.

Over the years, Lebanese built and bought luxury property opposite the port, a nearby Beirut Marina including restaurants, cafes and retail shops was built up, concerts were held, children rode their bicycles and workers went about their daily business, oblivious to the massive "bomb" waiting to explode.

At some point, someone battered open a door to Warehouse 12 and knocked a hole in one of its walls.

When is not known. It was reported when State Security inspected the site this summer. In a July 20 report, it warned that the warehouse's "Door Number 9 has suffered a blow in the middle, knocking it away from the wall enough to allow anyone to enter and steal the ammonium nitrate." It also noted the hole in the wall and pointed out that there was no guard at the warehouse, "making theft even easier."

The report to President Michel Aoun and then-Prime Minister Hassan Diab warned that thieves could steal the material to make explosives. Or, it said, the mass of material could cause an explosion "that would practically destroy the port." Kobaissi shared the report with the AP.

Aoun has been in office since 2016. After the explosion, he said the State Security report was the first time he'd heard of the dangerous stockpile. He said he immediately ordered military and security agencies to do "what was needed" - though he added he had no authority over the port.

After being criticized by rival politicians and on social media for not doing more, Aoun's office issued a further statement saying that his military adviser had immediately forwarded the State Security report to the Higher Defense Council, the top defense body in the country.

But a government official said security agencies had repeatedly sent warnings directly to the government.

"The same memo was sent roughly every year basically since that ship arrived, and it became clear the stuff wasn't moving. So, it was like a tradition and it wasn't marked as priority," the official told AP, speaking on condition of anonymity because he wasn't authorized to talk to the media.

Kobaissi, the investigative reporter, said all political factions in the country benefited from using the port for patronage, and most overlooked dubious dealings. He said many people knew about the initial warning by Skaff, including Hezbollah's former point man at the port.

Port and customs officials "are a gang, a mafia, appointed by a mafia gang that has come to office through an election process," Kobaissi told the AP.

He believes officials at the port were trying to find a legal cover to sell off the ammonium nitrate and skim off some of the money. He noted a similar scheme was run in the past when containers of confiscated asbestos were auctioned off. He said there were many instances of port officials profiting off impounded shipments, even keeping some goods - like Mini Coopers - for themselves.

Both the customs chief Daher and the head of the port, Hassan Koraytem, are among those detained in the wake of the explosion.

On the afternoon of Aug. 4, security officials say, three metalworkers who had been working for several days to weld the broken Door Number 9 of Warehouse 12 finished work and left the facility.

The cause of the original fire has still not been determined and is at the heart of the current investigation. Some have questioned whether the welding may have sparked stocks of flammable liquids used in making detergents, as well as tons of fireworks that were also being kept in Warehouse 12. Other possibilities such as sabotage are also being investigated. The metalworkers, who were hired to fix the door by the port authorities in response to the security report, have been detained for questioning, according to security officials.

Shortly after the 10 firefighters arrived at the port, they sent an urgent call back to headquarters, asking for reinforcements. Photos they sent from their mobile phones to their colleagues showed them trying to open the gate of Warehouse 12.

"When they called us, they said they are hearing the sound of fireworks," Beirut fire chief Nabil Khankarli told the AP.

No one told the emergency responders that dangerous material was stored in the warehouse. No port officials were even there to help them open the gate, Khankarli said.

A second team jumped into their vehicles and headed toward the port. All across the city, flames and the pillar of black smoke could be seen pouring into the sky, lit up by popping fireworks. Many residents would later report hearing a jet or a drone and presuming it was Israeli, since Israel sends reconnaissance flights over Lebanon on an almost daily basis. No evidence has yet emerged of warplanes.

There was an initial explosion, sending shredded debris into the air. That first blast, survivors would recount later, sent some who had been watching the fire scurrying for cover.

Twelve seconds later, at 6:08 p.m., the ammonium nitrate detonated in one of the biggest non-nuclear explosions ever recorded.

In an instant, a blast with the force of hundreds of tons of TNT sucked in the air - one video showed a luxury store window exploding outward from the suction, spraying a bride and groom taking their wedding video on the sidewalk outside - and then unleashed its power across the city.

It blew a crater nearly 200 meters (yards) wide out of the port where Warehouse 12 once stood, and seawater poured in to fill it. The port was leveled. A grain silo right next to the warehouse was shredded and sheared in half - though its massive bulk partially shielded sections of the city from the blast. For miles around, in people's homes and in shops and hospitals, windows were shattered, doors knocked off their hinges, ceilings or walls blown in a vicious whirlwind onto those inside.

Alaa Saad and his friends were out diving, about 2.5 kilometers (1.5 miles) off the coast of Beirut, when they started hearing noises from the direction of the port and saw the smoke. Was it fireworks? Ammunition?

"There were lots of flashes going off inside the smoke," he said. He heard some kind of eruption, like a volcano. "Something that was boiling very much," he said.

"Five seconds passed, and this is when I saw the cloud or the wave that was coming toward us at very high speed," he said. "It was insane speed. I could not even think if I wanted to jump in the water or stay on the boat."

Saad fell on the deck. A friend tumbled into the water.

"After that," he said, "I thought it was the end of Beirut or the end of the world or the war has started."

More than 6,000 people were injured, and at least 180 were killed - among them the 10 first responders. It would take days of searching before colleagues found all their bodies in the rubble.

Nearly three weeks later, theories abound. In the deeply polarized country, some have turned their suspicion to Hezbollah, which maintains a huge weapons stockpile in the country and dominates its politics. A member of the militant group was sentenced to six years in prison after he was arrested in Cyprus in 2015 in connection with the seizure of nine tons of ammonium nitrate at a house where he was staying.

An investigative team that includes Kobaissi, working with The Organized Crime and Corruption Reporting Project, found that the shadow owner of the Rhosus was actually a Cypriot who owed money to a Lebanese bank linked to Hezbollah - raising speculation that he brought in the ammonium nitrate for the group. The businessman, Charalambos Manoli, denied the report, insisting to the AP that he sold the ship in May 2012.

Others have peddled a theory that rivals of the group had sought to accrue the fertilizer for use as explosives in the war in neighboring Syria.

The documents show clear negligence and failure; the question of whether something more triggered the blast depends on an investigation that so far has seemed predictably slow and ineffectual.

The fire chief, Khankarli, is furious. So much destruction. So much bloodshed. All of it avoidable.

"We are waiting for the investigation," he said. "But what is gone cannot be recovered."

From AP