(NewsNation) — Frustrations have grown among parents as unanswered questions remain on how police responded to a school massacre in Texas that left 19 children and two adults dead.

Investigators say the gunman who opened fire on the elementary school in Uvalde, Texas, was inside the school for more than an hour while police were outside before he was killed by law enforcement. Authorities insist they were waiting on a tactical crew to arrive before charging into the school, while parents are angered they did not rush in to help the children.

Many parents say they frantically arrived on scene and urged police officers to help.

“They say they rushed in. We didn’t see that,” said Javier Cazares, whose daughter died in the shooting. “As soon as they heard that gunshot, they should have rushed in.”

Authorities say the gunman went inside the school, started shooting at officers and then barricaded himself inside a classroom, killing 21 people.

Many are left asking why would police wait to engage a mass shooter at a school.

When it comes to police response, former Tulsa Lt. Sticks Larkin explains there are generally different protocols for different situations.

“There’s a difference between an active shooter and somebody who is a hostage taker, or they’re armed and barricaded. In my time on the SWAT team, I got on just after Columbine, and one of the things that obviously drastically changed in law enforcement was how we respond to these type of deals.

“When you have an active shooter, police are showing up on scene and shots are still being fired. You know, I hate to use this term, but it’s a bad day to be a police officer. At that point, you have to go towards the shots fired, you have to step over victims, you can’t treat people, you have to go and try to stop the threat.

“When things change is when the shots have stopped being fired. If he is barricaded in a room, if he is in fact negotiating with one of the law enforcement officers out of there, that does change the tactics (of) how you’re going to attack this thing. But what you always do want to have at least close by when you do have somebody barricaded like that or negotiating, you have to have a hostage rescue team ready to go in at a moment’s notice,” Larkin explained.

Still, Ken Trump, president of National School Safety and Security Services, told the Associated Press the timeline raises questions.

“Based on best practices, it’s very difficult to understand why there were any types of delays, particularly when you get into reports of 40 minutes and up of going in to neutralize that shooter,” he said.

Texas public safety officials were grilled Thursday about changing stories and giving out conflicting information.

Reporters were initially told there was an exchange of gunfire outside the school between the gunman and a school resource officer. It turns out there was not a school resource officer there, which contradicts what the Texas governor said at a news conference.

Some say it’s difficult to have a clear picture of what happened in the days after a mass shooting.

“The information we have a couple of weeks after an event is usually quite different than what we get in the first day or two. And even that is usually quite inaccurate,” Michael Dorn, executive director of Save Havens International, told the Associated Press. For catastrophic events, “you’re usually eight to 12 months out before you really have a decent picture.”

Investigators are still facing mounting questions by community members following the deadliest shooting at a U.S. school since a gunman killed 20 children and six adults at Sandy Hook Elementary in Newtown, Connecticut, in 2012.

A Robb Elementary Memorial Fund account has been opened at First State Bank of Uvalde for the families of Robb Elementary, according to the school district. Donations will be accepted at all FSB branches.

Checks should be payable to the “Robb School Memorial Fund” and can be mailed to 200 E. Nopal St. Uvalde, TX 78801. Donations through Zelle can be sent to robbschoolmemorialfund@gmail.com, according to the school district.