(ST LOUIS, Mo.) -- The family of the 19-year-old suspect accused of opening fire in a St. Louis, Missouri, high school had recently removed the firearm used in the deadly school shooting from their home, but the teen somehow got ahold of it again, authorities said Wednesday.
One student and one teacher were killed in the Monday morning shooting at Central Visual and Performing Arts High School. Several others were injured.
The suspect, Orlando Harris, who police said was armed with an AR-15-style rifle and more than 600 rounds of ammunition when he "forced entry" into the building, died during an exchange of gunfire, according to St. Louis Police Commissioner Michael Sack.
Sack told reporters during a news conference Wednesday that the family had previously contacted the department about a firearm discovered in the home.
On Oct. 15, police responded to a domestic disturbance at Harris' home because the "suspect's mother had located a firearm in the home and wanted it removed," St. Louis Sgt. Charles Wall said in an update Wednesday evening.
Police determined that Harris legally possessed the gun, and a "third party known to the family" took the gun so it was no longer stored in the home, Wall said.
"While it is not yet clear when or how the suspect came to be in possession of the firearm after this incident, we can confirm that the firearm involved in this incident is the firearm used in the shooting Monday," Wall said.
Harris -- who graduated from the high school last year -- had been seeing mental health professionals, Sack said Wednesday, and his family had him committed on some occasions.
Whenever Harris' family "noticed him, kind of, stepping out of line ... they always worked to try and get him back on his medication, back into therapy, whatever it is that he needed," Sack said.
The family -- who has been "fully cooperative" with police -- appears to have "done everything they could have possibly done" to help Harris, Sack said, "but sometimes that's not enough."
Wall added that "this tragic incident occurred despite their best efforts."
Harris "felt isolated and alone" and "there was a disconnect between him and what he felt was the school community," Sack said.
The school "had always been the target," he said.
Sack said Tuesday that Harris left behind a notebook with writings about his desire to "conduct this school shooting."
According to Sack, Harris wrote: "I don't have any friends, I don't have any family, I've never had a girlfriend, I've never had a social life."
Sack said Harris called himself an "isolated loner," which Harris called a "perfect storm for a mass shooter."
Harris' family would search his room on occasion but the family was not aware of his notebook, Sack said.
It's not yet clear when or how Harris bought the gun, Sack said.
ABC News' Will Gretsky contributed to this report.