They rush straight to the gunfire.
That's how the April 1999 massacre at Columbine High School -- where two young men killed 13 people -- shaped the way law enforcement respond to active shooter incidents such as Wednesday's deadly rampage in Parkland, Florida.
"It changed everything," said James Gagliano, a retired member of the FBI's elite hostage rescue team.
"Prior to Columbine, nobody understood what the term 'active shooter' meant."
Nearly two decades ago, a pair of Littleton, Colorado, students named Dylan Klebold, 17, and Eric Harris, 18, carried out a killing spree at Columbine that law enforcement experts called a watershed event in the response to active shooters.
Within 13 minutes of the first 911 call, Klebold and Harris fatally shot 12 students and a teacher and wounded 23 other people before killing themselves with gunshot wounds to the head. SWAT teams entered the school 47 minutes after the gunfire erupted.
An exhaustive FBI review of the police response at Columbine led to a more rapid response strategy during active shooter situations, according to Gagliano.
The "stark and important" lessons learned from the Columbine response, which was widely criticized, may have helped save lives Wednesday at Marjory Stoneman Douglas High School in Parkland, Gagliano said. Before the Colorado shooting, responding officers would set up a secure perimeter around the crime scene before even thinking about moving on the suspect.
In Parkland, authorities say Nikolas Cruz, 19, fatally shot 17 people at his former school before blending in with the students and staff rushing out of the school building. He was arrested in a neighboring community later in the day.
"Nowadays, what we do is go to the sound of the guns," Gagliano said. "You get one, two, three, four people together. We're trained. We use particular formations."
Gagliano called it a "heterogeneous group" of first responders that could include local, state and federal agencies.
"You're going to the sound of the guns," he said. "The No. 1 goal is to interdict the shooter or shooters. In the old days, you took land. You went in. You clear the room. Then you slowly and methodically move to clear the next room. In this instance ... get to the shooter as quickly as possible and that's what they clearly did here."
The tactic, known in law enforcement circles as rapid deployment involving the first officer at the scene, began in earnest after the Columbine shooting.
More than half of mass shooting incidents are still in progress when officers arrive on the scene, with 75% requiring law enforcement to confront the shooter before the threat ends, Katherine Schweit, a former senior FBI official, wrote in a 2013 FBI Law Enforcement Bulletin article.
A study of 35 active shooter incidents during 2012 found that 37% ended in less than five minutes and 63% in less than 15 minutes, according to Schweit. The average active shooter incident lasts 12 minutes.
The lessons learned from Columbine led the US Justice Department and other federal agencies to partially fund an active shooter program known as Advanced Law Enforcement Rapid Response Training, or ALERRT, Schweit wrote.
The training, which was developed by the San Marcos, Texas, Police Department and the Hays County, Texas, Sheriff's Department and adopted by Texas State University in San Marcos, includes a 16-hour course that "prepares first responders to isolate, distract and end the threat when an active shooter is engaged," according to Schweit.
Since its creation in 2002, more than 105,000 law enforcement officers have been trained through the ALERRT program.
"Some of the lessons learned after Columbine, we called it post-Columbine, were stark and important and, I believe, saved a number of lives," Gagliano said.