Former Broward County sheriff’s deputy Scot Peterson, shown testifying in a 2015 Broward County School Board meeting, was the school resource officer at Marjory Stoneman Douglas High School. (Broward County School Board)

In American policing, confronting active shooters is divided into two eras: before Columbine, and after Columbine. Before the 1999 shooting at Columbine High School in Colorado, police strategy was to wait for the SWAT team to arrive and then attack en masse with precise force. But after the two shooters in Columbine roamed the school for nearly 50 minutes, killing 13 and wounding 21, the police approach changed: Enter now. Whoever is there with a gun, whether a school resource officer or the first patrol officer to arrive, should go after the shooter.

“We teach that the first priority when you come on scene is to stop the killing,” said Pete Blair, executive director of the Advanced Law Enforcement Rapid Response Training Center at Texas State University and one of the nation’s top experts on active shooter training. “The number one driving force is gunfire. If there’s gunfire, we teach the officers to isolate, distract and neutralize. We want people to go directly to the sounds of the gunfire.”

But as a teenager armed with an AR-15 rifle was blasting away at students and teachers inside Marjory Stoneman Douglas High School in Florida last week, school resource officer Scot Peterson remained outside and did not engage the shooter, Broward County Sheriff Scott Israel said. Fourteen students and three teachers were killed. Peterson, who spent 27 of his 32 years in law enforcement as a school resource officer, has resigned, Israel said.

Peterson should have “went in,” Israel said. “Addressed the killer. Killed the killer. … There are no words. I mean these families lost their children. We lost coaches,” Israel said.

“I’m in shock and I’m outraged to no end that he could have made a difference in all this.” Broward County Public Schools Superintendent Robert W. Runcie the South Florida Sun Sentinel. “It’s really disturbing that we had a law enforcement individual there specifically for this reason, and he did not engage. He did not do his job. It’s one of the most unbelievable things I’ve ever heard.”

Facing a gunman with a high-powered weapon is a life-altering call, and officers don’t always charge in despite their training. In Las Vegas last October, two armed sheriff’s deputies and several armed security officers working for the Mandalay Bay Resort and Casino reached the 32nd floor within six minutes of the opening fusillade by gunman Stephen Paddock, a report by the Las Vegas police showed. But the deputies and officers, unsure of what was behind the hotel room door, did not attack Paddock’s room as he continued to pour shots down into a country music concert for another four minutes, the report found. Paddock then committed suicide, having killed 58 people at the concert.

Blair said that situation was different in that officers couldn’t see their target, didn’t know if hostages were involved, and couldn’t just fire blindly down a hallway. The gunfire ended four minutes after the police arrived, and deputies then focused on evacuating the vicinity and approaching a suite which appeared to be booby-trapped, with camera wires leading from a cart into the room.

President Trump has proposed arming teachers who could fight back in the complicated chaos of an active-shooter situation. Though the two biggest national groups of educators immediately declared their opposition to this, in Texas it is already in effect. Legislators in 2013 created a “school marshal” position for teachers who already have a concealed carry permit, “to prevent the act of murder or serious bodily injury on school premises.” The marshals must complete an 80-hour course conducted by the state law enforcement commission, and the location of districts and teachers involved in the program is kept strictly confidential, so that potential attackers don’t know which schools have armed teachers in them. More than 170 school districts now have such marshals, the Corpus Christi Caller-Times reported.

Among those who think this is a bad idea is the National Association of School Resource Officers, which describes itself as “the world’s leader in school-based policing.” Executive Director Mo Canady said that law enforcement officers responding to an active shooter could mistake a teacher for an assailant, that teachers might not maintain their training and accuracy, and that teachers might not be equipped to assess whether to fire in a crowded school.

“Teachers don’t want to be armed, we want to teach,” American Federation of Teachers President Randi Weingarten said in a statement. “We don’t want to be, and would never have the expertise needed to be, sharp shooters; no amount of training can prepare an armed teacher to go up against an AR-15.”

Peterson, 54, was expected to be the first line of defense at Stoneman Douglas High. He had been the school resource officer there since 2009, and also served the same function at Atlantic Technical College in Coconut Grove, Fla., according to a public statement he made to the Broward County School Board in 2015. He testified on behalf of a program to have such officers live on campus, and described once capturing four burglars single-handedly.

“We’re all here for the same goal,” Peterson said then. “To protect our kids, to protect our property.”

Peterson could not be reached for comment Friday.

Records show Peterson was familiar with Nikolas Cruz, the 19-year-old student who had been expelled from Stoneman Douglas  for behavioral problems. As Cruz approached the building, a staffer spotted him and radioed that he was nearing the school. But Israel said video showed Peterson stood outside the building for “upwards of four minutes” while Cruz was shooting. “What I saw was a deputy arrive … take up a position and he never went in,” the sheriff said.

Some experts noted that police departments have differing policies on how many officers are needed before confronting an active shooter, and the Broward County Sheriff’s Office refused to release its policy Friday. Some departments do not permit an officer to approach alone. But Israel’s comments indicated he felt that Peterson didn’t follow department protocol.

“Of course the officer should have gone in,” said Chris Grollnek, a retired police officer and active shooter prevention expert. “But the rest of the world needs to realize, when you pin a badge on a person, there’s no magic cape that goes with that. You don’t become bulletproof. You’re still human.”

Grollnek noted that most mass shootings are over in about six minutes, which isn’t enough time for a typical police response. He said schools need a revamped approach to shootings, such as rehearsing evacuations in the same way fire drills are rehearsed. “Over 90 percent of fatalities from an active shooter die because they’re trying to hide,” Grollnek said. “We need to teach everyone to ‘G.O.’ Get out.”

Canady said school resource officers tend to be passionate about working with students. His association trains officers in dealing with active shooters.

“We’re very specific,” Canady said. “If you are in a school and you hear shots fired, you can either wait for backup to arrive, and the shooter keeps killing people, or you can go direct to the threat, use your tactics and stop it. It’s a tough notion, but we’re tough people. We’re police officers, we’re sheriff’s deputies. They take it seriously, keeping students safe. Once it does happen, you’ve got to be able to react and stop it. That’s what we encourage in the basic course.”

Blair said officers are trained to consider their own safety. “If the scene is still hot when officers arrive,” he said, “one out of four officers is shot. That is extremely dangerous for them. At the same time, you’re the one who has a weapon, body armor and training, and these people don’t.”

Chuck Wexler, executive director of the Police Executive Research Forum which advises big city police chiefs on policy and procedures, said, “Philosophically, since Columbine the principle is, you hear active fire, you’re expected to take action. … Chiefs recognize they are putting their officers in harm’s way by doing this. But the consequences of them not acting are far worse.”