The suspect in a Michigan school shooting possibly had the gun with him when he met with school officials that morning.
The meeting was called to address 15-year-old Ethan Crumbley's "concerning behavior," police said.
Oakland County Prosecutor Karen McDonald told CNN she is still considering charging the suspect's parents.
The 15-year-old suspected gunman in a Michigan high school shooting that left four students dead and several others injured, likely had the weapon in his possession during a behavioral meeting with his parents and school officials just hours before the rampage, the county prosecutor said.
Oakland County Prosecutor Karen McDonald told CNN's Anderson Cooper there was a "very strong possibility" that sophomore Ethan Crumbley, who has been charged in the crime, was carrying the semiautomatic handgun in his backpack during the face-to-face meeting with his parents and administrators, which was called to address his "concerning behavior."
"Unfortunately he was allowed to go back to class," McDonald said. "We now know he had a weapon with him at that time, and that is simply tragic."
McDonald said students at Oxford High School were not using lockers this semester due to COVID-19, and thus, only had access to their backpacks throughout the day.
CNN reported that two teachers flagged Crumbley's concerning behavior, which prompted the meeting.
Crumbley has been charged with four counts of first-degree murder, which implies premeditation, one count of terrorism causing death, seven counts of assault with intent to murder, and 12 counts of felony firearm possession.
Justin Shilling, 17, Tate Myre, 16, Hana St. Juliana, 14, and Madisyn Baldwin, 17, were killed in the shooting. Seven others, including a teacher, were injured.
McDonald on Thursday evening reiterated her previous statement that her office is considering charging Crumbley's parents in the case as well. She said she would announce an update on the matter on Friday.
"Gun ownership is a right, but with that right comes responsibilities and duties," she told Cooper. "I'm saying [gun owners] should be held accountable to what happens to that gun. You must secure it safely. You must make sure it doesn't end up in the hands of someone who intends to do harm."
Authorities on Tuesday said Crumbley used a pistol his father had purchased on Black Friday, identified as a 9mm Sig Sauer SP 2022 pistol.
McDonald told reporters earlier this week that evidence suggested the suspected shooter had started planning "well before" the shooting.
"I am absolutely sure after reviewing the evidence that it isn't even a close call," she said. "It was absolutely premeditated."
During her CNN appearance, McDonald also said there is a piece of evidence that has not yet been released that proves the shooting "probably" could have been prevented. She said that information would also be publicized on Friday.
Neither the Oakland County Prosecutor's Office nor the Oakland County Sheriff's Office immediately responded to Insider's request for comment. Current attorney information for Crumbley is not available.