COLORADO SPRINGS, Colo. — Authorities scrambled Sunday to determine what led a gunman to enter a Colorado Springs LGBTQ nightclub just before midnight Saturday and open fire, leaving five people dead and 25 injured.

Colorado Springs Police Chief Adrian Vasquez identified the gunman as Anderson Lee Aldrich, 22, who was in custody and being treated for injuries after patrons confronted him and stopped the shooting, which came during Transgender Awareness Week.

El Paso County District Attorney Michael Allen said investigators were looking into all possible motives and whether the attack should be prosecuted as a hate crime. But charges against the suspect “will likely include first-degree murder,” he said.

The shooting rekindled memories of the 2016 massacre at the Pulse gay nightclub in Orlando, Florida, that killed 49 people. Colorado has experienced several mass killings, including at Columbine High School in 1999, a movie theater in suburban Denver in 2012, and at a Boulder supermarket last year.

It was the sixth mass killing this month and came in a year when the nation was shaken by the deaths of 21 in a school shooting in Uvalde, Texas.

Here’s what we know about the shooter in Saturday’s deadly attack:

Authorities say assailant acted alone; reported prior interaction with police

Police said they believe Aldrich used a “long rifle” and acted alone during the shooting — but so far have offered few details on his background. The club owners did not know the suspect and were not aware of any recent threats toward the club, they told the New York Times.

Two residents of an apartment complex where records show Aldrich possibly lived once didn’t know him. One resident said police had been there asking questions.

A man with the same name and age as Aldrich was arrested in 2021 after his mother reported he threatened her with “a homemade bomb, multiple weapons, and ammunition,” according to authorities.

An El Paso County Sheriff’s Office release last year said that officers evacuated a portion of the area and eventually persuaded Aldrich to surrender. No explosive devices were found.