Former FBI criminal profiler Mary Ellen O'Toole says shooters like the suspect who killed 19 children and two teachers at an Uvalde, Texas, elementary school on Tuesday are hard to identify early on. 

"With the shooter, some of the early terminology calling him ‘mentally ill’ or [saying] his behavior was driven by mental illness can be misleading because they aren’t always mentally ill," she said of mass shooters. "[S]aying so attempts to show that this was something nobody could’ve done anything about. Risk factors were present in this offender's background. Paying attention to those warning signs ahead of time can prevent the next shooting."

Salvador Ramos, the now-deceased 18-year-old suspect in Tuesday's shooting, apparently shared messages on Facebook about shooting his grandmother and having plans to shoot up an elementary school, according to Texas Gov. Greg Abbott. The suspect also apparently posted disturbing images of firearms and firearm components on Instagram ahead of the shooting. 

Texas school shooting police officer lights candle

A law enforcement personnel lights a candle outside Robb Elementary School in Uvalde, Texas, Wednesday, May 25, 2022.  (Jae C. Hong)

Ramos legally purchased AR platform rifles on May 17, 2022 and May 20, 2022, according to a briefing from Texas Sen. John Whitmire on Wednesday. One of the guns was found in Ramos’s car outside the school and the other one was found with his body inside the school. Ramos also purchased 375 rounds of 5.56 ammunition, Whitmire and Texas State Sen. Roland Gutierrez told the Austin American-Statesman. 

TEXAS SCHOOL SHOOTING: LIVE UPDATES

Shooter "red flags" include "evolving" behavior and thoughts about "acting out violently," which can occur "days, weeks and months ahead of an attack," even online, O'Toole said.

"Additional risk factors can include things like some kind of depression, interpersonal violence, substance abuse or a triggering event, like they don’t graduate high school, so now what they were planning kicks into high gear," the profiler explained.

Law enforcement on scene at Robb Elementary School in Uvalde, Texas

Law enforcement work the scene after a mass shooting at Robb Elementary School in Uvalde, Texas, on Tuesday, May 25. (Jordan Vonderhaar)

O'Toole also described the time that elapsed between the moment the shooting started and the moment law enforcement arrived at the school as "concerning."

UVALDE, TEXAS SCHOOL SHOOTING: TIMELINE OF MASSACRE THAT LEFT AT LEAST 19 CHILDREN, 2 TEACHERS DEAD

"I am by no means implying they did anything wrong," the former profiler said. "There could’ve been things we don’t know about, like more threats, negotiations, etc. I wouldn’t want to say anything sinister happened, but it is curious to me that there could’ve been this period of time where law enforcement didn’t enter. Bits and pieces are coming out that he was in there for 40 minutes. If that is true, it is concerning."

The suspected gunman could have been in the school for 40 minutes to an hour, according to Texas Department of Public Safety Director Steve McCraw.

Police Texas school shooting

Police walk near Robb Elementary School following a shooting, Tuesday, May 24, 2022, in Uvalde, Texas. (Dario Lopez-Mills)

"Right after Columbine, we learned a lot of lessons," O'Toole said. "First responders don’t wait. They go in and attempt to overwhelm and kill the shooter because after what happened in Columbine, where the shooters were allowed to continue shooting. We will learn from the timeline as it comes out. I’d never second-guess first responders on the ground. They had to make the best decision at the time."

PHOTOS: FAMILIES, VICTIMS OF TEXAS SCHOOL SHOOTING

The Uvalde shooting was the most deadly elementary school shooting since Connecticut shooting suspect Adam Lanza killed 20 students and six teachers at Sandy Hook Elementary School before shooting himself in 2012. 

Both deceased suspects could have been stopped by their guardians, O'Toole suggested. In Ramos' case, his grandparents may have been able to prevent the tragedy, and in Lanza's case, his mother may have been able to stop his actions.

State Police are on scene following a shooting at the Sandy Hook Elementary School in Newtown, Conn., about 60 miles (96 kilometers) northeast of New York City, Friday, Dec. 14, 2012. An official with knowledge of Friday's shooting said 27 people were dead, including 18 children. (AP Photo/Jessica Hill)

Connecticut State Police are on scene following a shooting at the Sandy Hook Elementary School in Newtown, Conn. (Jessica Hill)

"Did they not want them to interfere with their plans?" O'Toole asked, adding that both suspects also chose "the most vulnerable" victims as their targets "with lethal weapons designed for the maximum lethality."

An investigation into Tuesday's shooting remains active and ongoing. An agent with the Border Patrol Tactical Unit (BORTAC), is believed to have fatally shot the suspected gunman.

CLICK HERE TO GET THE FOX NEWS APP 

On Wednesday, the Uvalde Police Department Chief Daniel Rodriguez released a statement saying its officers responded to reports of a shooter "within minutes," alongside school district officers. Two responding officers sustained gunshot wounds.

"This is an ongoing investigation that is being led by the Texas Rangers. I understand questions are surfacing regarding the details of what occurred. I know answers will not come fast enough during this trying time, but rest assured that with completion of the full investigation, I will be able to answer all the questions that we can," Rodriguez wrote.

Fox News' Jon Brown and Brie Simson contributed to this report.