A former student who opened fire inside a Colorado high school in 2019 was convicted of murder on Tuesday for killing a classmate who charged the gunman to save his classmates' lives.

A jury in Douglas County, Colorado took less than a day to find Devon Erickson guilty of first-degree murder in the shooting rampage at Science, Technology, Engineering and Math School in Highlands Ranch, Colorado, on May 7, 2019.

Erickson carried out the shooting alongside a friend, Alec McKinney, who pleaded guilty last year to murder, attempted murder, conspiracy and weapons charges.

McKinney, who was 16 at the time of the shooting, was sentenced to life in prison plus 38 years but as a juvenile can seek parole after 40 years.

McKinney told investigators following their arrest that he and Erickson carried out the shooting as revenge over being bullied by classmates because he is transgender.

During Erickson's trial, McKinney testified on behalf of the prosecution, telling the court that it was Erickson who had picked the classroom to target.

Erickson, 20, faces life without the possibility of parole for the murder conviction when he is formally sentenced on September 17. He was also found guilty of numerous related offenses.

Prosecutors presented evidence at the three-week trial that Erickson shot Kendrick Castillo, who along with two other students rushed Erickson when he burst into a classroom, and opened fire with a handgun.

Castillo, 18, died from a gunshot wound to the chest.

Seven students were wounded by bullets fired by McKinney and Erickson during the rampage, and an eighth was accidentally shot by a security guard during the chaos.

Defense lawyers argued that Erickson should be convicted of lesser charges because he was a longtime drug abuser. They claimed he took part in the shooting because he feared McKinney.