The Uvalde massacre began after the 18-year-old gunman entered the school through a door that could only be locked from the outside then got inside a classroom that had a busted lock, experts testified Tuesday.

Securing doors has long been a focus of school safety drills, and the inability to do so during the May 24 attack that left 19 children and two teachers dead is raising alarms among experts and politicians.

When doors are not secure, "your first step, your first line of defense has now been eliminated," said Ken Trump, the president of the National School Safety and Security Services.

State Sen. Paul Bettencourt said unlockable doors make lockdowns and shooter training worthless, adding that there was "zero obstacle to the shooter."

Questions about how the shooter entered Robb Elementary and what happened at multiple doors have been a big part of the changing information about the attack.

WHAT HAPPENED WHEN THE GUNMAN REACHED THE OUTSIDE DOOR?

State police initially said the gunman entered the school through an exterior door that had been propped open by a teacher.

Days later, state police retracted that statement to make it clear that the teacher closed the door. But somehow it didn't lock.

Nearly a month after the rampage, Col. Steve McCraw, director of the Texas Department of Public Safety, further amended what his agency's investigation shows: The teacher did close the door, but unbeknownst to her, it could be locked only from the outside.

The gunman "walked straight through," McCraw said Tuesday in blistering testimony at a state Senate hearing in Austin.

Ronald Stephens, executive director of the National School Safety Center, said he was "astonished" that the exterior door could only be locked from the outside. He likened it to a house that could only be locked from the outside.

"Shouldn't the security of the school be as safe as the security of your home?" he asked.

Experts did not explain during the hearing why the school's exterior door locked from outside. Robb Elementary is an older building, constructed in 1955.

WHAT HAPPENED WHEN THE GUNMAN REACHED THE CLASSROOM DOOR?

Once inside the school, the shooter then entered a classroom though a door that was designed to be locked from the outside, according to McCraw, who also said a teacher reported before the shooting that the lock was broken.

"This is ridiculous and it's inexcusable," McCraw said of the fact that the classroom door could not be locked from inside.

Stephens and Trump also raised alarms about the fact that the door was broken, describing it as a maintenance issue.

"That's about how you manage and maintain school property in a responsible way," Stephens said.

McCraw also disclosed Tuesday that despite the door being unlocked, there was no indication officers tried to open it during the standoff. He said police instead waited for more than an hour for a key.

"To me," Stephens said, "there's just a whole cascade of apparent failures that took place in this particular situation."

WHY DID THE DOORS LOCK FROM THE OUTSIDE?

Many schools designed in the 20th century featured classroom doors that locked from the outside, allowing the teacher or administrator to lock up as they left for the day, Todd Ferking explained in an email. Ferking is a design leader for DLR Group, an architecture firm that specializes in school design.

"Locking from inside the classroom may not have been a popular option out of concern that students could lock the teacher out," he said.

The Columbine tragedy led to an evolution in school construction, he said, with most new classrooms designed to provide locking from inside via a key or thumb turn.

Today, it also is general practice that all exterior doors are locked during school hours, except during drop-off and pick-up, he said.

HAVE THERE BEEN PROBLEMS BEFORE?

At Sandy Hook Elementary School, the doors of the two classrooms where all 20 children were killed in the 2012 massacre, along with their teachers, could only be locked from the hallway with a key.

Some victims' families have said lives could have been saved if teachers had been able to lock classroom doors from the inside, and they questioned whether two teachers who were killed in the shooting, Victoria Soto and Lauren Rousseau, even had access to keys.

Another teacher who could not get a classroom door locked told investigators that she looked into the hallway, saw a janitor who yelled at the gunman to leave and motioned to the janitor to lock her door.

Sandy Hook Elementary was built around the same time as Robb Elementary, in 1956.

Mo Canady, the executive director of the National Association of School Resource Officers, spoke publicly about the importance of being able to secure doors after Sandy Hook. He lamented that it was still an issue a decade later.

"That school," he said of Robb Elementary, "I can promise you, is not the only one in this country that you can't lock the doors from the inside."

Such doors, he said, occasionally are spotted during assessments of buildings, particularly older ones. He described it as "unacceptable" and urged schools to fix busted doors and retrofit doors that only lock from the outside while students are on summer break.

"The basics are so important, and if your school district does not have doors that will allow the teachers to secure those in a lockdown, that's a priority," he said. "Those things really can and do save lives."

WHAT STEPS ARE RECOMMENDED?

State and federal panels charged with reviewing individual mass shootings have repeatedly advised schools to limit access by locking exterior doors, as well as forcing visitors to enter through a secure door and requiring teachers to lock classrooms while classes are in session.

Teachers and students drill for how to respond.

"Lock the door, turn off the light. Get the kids and staff into a hard corner, meaning not in the direct line of sight of the window where somebody can shoot through, and be quiet," Trump said.

He said those actions can "absolutely" save lives.

Exterior doors "keep the threat out of the building. Your next layer inside are your locked doors to your classrooms, officers and work areas. All of these layers are intended to buy time," he said.

He said shooters' adrenaline is going and they know that - in most cases - their time is limited.

"They're not going to spend a ton of time trying to get in a locked door where they may or may not know if somebody's inside, if the kids are quiet and out of sight. They're going to continue to move on to where they have a more accessible target."

From The Associated Press