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Students from STEM School Highlands Ranch walk from their school entrance, with their hands on their heads, after a shooting that killed one student and wounded eight others on May 7, 2019, in Highlands Ranch, Colorado. (John Leyba, Special to The Colorado Sun)

The shootings at the STEM School at Highlands Ranch stood out in one important respect from other school shootings, according to experts who study the psychology of mass shooters.

Two suspects are accused of acting in coordination on Tuesday. The vast majority of school shootings are committed by a lone gunman.

Only two team shooters are known to have killed fellow students at schools, said John Nicoletti, a Lakewood-based expert in crisis intervention and trauma recovery. Nicoletti helped guide the response to the mass shootings at both Columbine High School and the Aurora Theater.

Mitchell Johnson and Andrew Golden were minors at the time — 13 and 11 respectively — when they killed five in a mass shooting at Westside Middle School in Jonsesboro, Arkansas, in 1998. They were apprehended and convicted. Eventually, they were released from custody.

The Columbine High School shootings of 1999 by two students — one 17 years old and the other 18 — is the only other team mass shooting at a school, according to Nicoletti. Thirteen victims were killed in that attack.

Nicoletti said other team shooters have plotted attacks, but usually they are disrupted by law enforcement because one of the participants ends up broadcasting intentions ahead of time.

That happened in October when two young girls at Bartow Middle School in Bartow, Florida, were taken into custody after they were discovered in the bathroom of the school with several knives on them. The girls, aged 11 and 12 at that time, had been plotting to kill at least 15 of their fellow students, law enforcement authorities said at the time of their apprehension.

Authorities said one of the students had warned another student to steer clear of an area of the school to avoid being harmed.

Two 16-year-old girls also planned an attack at Mountain Vista High School in Douglas County in 2015, but authorities successfully disrupted their plans after one of the girls wrote of the details in her journal, which was discovered.

In Colorado, school districts and schools throughout the state have put in place threat assessment systems that are meant to disrupt plans for violence. The number of threat assessments has surged in the state’s school districts, with some districts seeing a tenfold increase in recent years, a recent investigation by The Colorado Sun found.

After the Columbine shootings in 1999, the FBI and the Secret Service each conducted studies of school shootings. Those studies did not find there was a common “profile” of a school shooter, but the research did reveal that almost all students who end up killing had told someone of their intentions ahead of time.

Updated at 6:15 a.m. May 8, 2019 to reflect new information about the gender of one of the STEM School suspects.

Updated at 1 p.m. May 8, 2019 to correct the year in which two teen girls were plotting an attack at Mountain Vista High School. It was 2015.