(NewsNation) — Frustrated onlookers urged police officers to charge into the Texas elementary school where a gunman’s rampage killed 19 children and two teachers, witnesses said Wednesday, as investigators worked to build a timeline of the massacre that lasted upwards of 40 minutes and ended when the 18-year-old shooter was killed by a Border Patrol team.

“Go in there! Go in there!” nearby women shouted at the officers soon after the attack began, said Juan Carranza, 24, who saw the scene from outside his house, across the street from Robb Elementary School in the close-knit town of Uvalde. Carranza said the officers did not go in.

“The police should have responded much quicker because he was out in the streets shooting,” former CIA officer Bob Baer said during an appearance Thursday night on NewsNation’s “Banfield.”

Once the shooter is in the classroom, it’s a different situation, according to Baer.

Baer, a frequent law enforcement analyst and author of the “The Fourth Man: The Hunt for a KGB Spy at the Top of the CIA and the Rise of Putin’s Russia,” is skilled in multi-agency operations and assessing responses.

“The police are not trained to do an assault. They’re not equipped to do an assault. They’re instructed to wait for the SWAT team, which clearly would have been too late,” Baer said.

Months of training is required for a cop to take on an active shooter with an assault rifle, according to Baer.

“He’s (a cop) got to come in with explosives, flashbang grenades, and all the rest of it. They’re just not equipped to do it.”

“They weren’t prepared. And the schools, at the end of the day, should be completely locked down. There should be no back entrances. The front entrance should be controlled by a guard, airlocks,” Baer added.

Regardless, though, Baer says that smaller schools are easy targets.

“They’re trying to gather so much information at one time, and sometimes that information just doesn’t come out accurate,” Vernon Stanforth, president of the National Sheriffs Association said on “NewsNation Special Report.”

“It’s important that the law enforcement be upfront, honest. But do it in a calculated sense, where they have the facts, not speculation. That’s critical. The people need to know the facts,” Stanforth said.

The Associated Press contributed to this report.