The
Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) was at the scene following the shooting. A spokesperson said the Bureau recovered the 9mm caliber handgun used in the shooting and was running a trace on it. Police and the
Federal Bureau of Investigation (FBI) spoke to the perpetrator's family and students to attempt to find a motive. The perpetrator's family cooperated with law enforcement throughout the investigation. Chief Barnes said he was aware that the alleged
manifesto, claimed to be from the shooter, was circulating on the internet, but he could not confirm its authenticity. He also said that he has requested that the ATF expedite their investigation into the shooter's whereabouts and method of obtaining the firearm. He was unsure if her parents owned or had possession of the weapon. On December 17, Barnes told news reporters that investigators are looking into a "combination of factors," including the possibility that Rupnow had been bullied. While searching Rupnow's home, officers found a cardboard model of the school, maps, and a schedule of the attack, beginning with her opening fire at 11:30 a.m., "wiping out" the first and second floors of the school, and then ending the attack at 12:10 p.m. with the note "ready 4 Death."
Potential conspirators and copycats On December 18, 20-year-old Alexander Paffendorf from
Carlsbad, California, was detained by FBI agents in connection with the shooting. He was suspected of coordinating a
mass shooting at a government building in conjunction with Rupnow. Allegedly, he told Rupnow that he "would arm himself with explosives and a gun and that he would start shooting in a government building." It is currently unclear how Paffendorf and Rupnow knew each other; the FBI declined to comment on this situation. As a result of his statements and admissions, a gun violence emergency protective (restraining) order approved by a San Diego County judge was served on Paffendorf. The order requires Paffendorf to turn over firearms and not to possess any guns while the order is in effect. On January 3, 2025, Paffendorf appeared via video in the
San Diego County Superior Court for a restraining order hearing. He apologized for his connection to the shooting, expressed regret for his actions, and said he was willing to face the repercussions of his actions. The judge postponed the hearing until April 4 because Paffendorf is under criminal investigation and did not yet have an attorney. On April 28, a three-year restraining order was granted. Rupnow followed the
X account of 17-year-old student Solomon Henderson, who went on to
open fire at
Antioch High School in
Nashville, Tennessee, on January 22, 2025, killing one student and injuring another before committing suicide. On April 29, 2025, 22-year-old Damien Allen from
Loxahatchee, Florida, was arrested by police for plotting seven separate mass shootings at seven different locations, including at least one police station and a church. He was found to have exchanged messages with Rupnow prior to her death. Allen was also found to be in possession of 18 firearms and over 12,000 rounds of ammunition. On August 27, 2025, 23-year-old Robin Westman carried out
a mass shooting at the
Church of the Annunciation in
Minneapolis, Minnesota, killing two children and injuring 30 other people before committing suicide. She had written Rupnow's last name on the gun used in the shooting. On September 10, 2025, 16-year-old student Desmond Holly
opened fire at
Evergreen High School, injuring two students before committing suicide. He idolized Rupnow online and took a photo of himself imitating her before the shooting. In November 2025, a 15-year-old boy was detained in
Kenosha, Wisconsin, after he made a threat on TikTok against a local school, timed to occur on the same date as the ALCS shooting. ==Reactions==