A fourth student has died following Tuesday afternoon's shooting at a Michigan high school.
Justin Shilling, 17, died at about 10:45 a.m. Wednesday in the wake of the shooting at Oxford High School in Oxford Township, sheriff's officials said. Three other students, ages 14 to 17, died Tuesday. Seven people, including a teacher, were injured.
The suspected gunman, 15-year-old Ethan Crumbley, was taken into custody and is being charged as an adult, Oakland County Prosecutor Karen McDonald said at a news conference Wednesday.
There's no indication that the victims were specifically targeted, Oakland County Sheriff Michael Bouchard said Wednesday.
McDonald said she is confident prosecutors can prove the shooting was premeditated "well before the incident."
A law enforcement official told ABC News that investigators are actively pursuing information that, Monday night, an undetermined number of students appeared to see a Snapchat video warning of a shooting on Tuesday. Some students who saw the video stayed home from school, though no calls were placed to police regarding the video, the official said.
During Crumbley's first court appearance Wednesday afternoon, Lt. Tim Willis of the Oakland County Sheriff's Office confirmed that videos were recovered from the suspect's phone. Willis said they included one video by Crumbley the night before the shooting in which he allegedly "talked about shooting and killing students the next day at Oxford High School."
Investigators also recovered a journal from Crumbley's backpack that allegedly detailed a desire to kill students.
According to the sheriff's office, "the suspect had been involved in a meeting over behavior issues the prior day and the day of the shooting."
"Nothing of concern was noted in his school file prior to the first meeting," the sheriff's office said. "There are also no documented cases of bullying of the suspect with the school."
Crumbley has been charged with one count of terrorism causing death; four counts of first-degree murder; seven counts of assault with intent to murder; and 11 counts of possession of a firearm in commission of a felony, she said. Additional charges are possible, McDonald said.
A judge entered a not guilty plea for Crumbley at his arraignment. He will be moved to Oakland County Jail and held in isolation with bond, the judge said. His next court appearance is scheduled for Dec. 13.
The teen allegedly took his father's semiautomatic handgun, a 9 mm Sig Sauer pistol, with him to school, officials said.
Crumbley allegedly entered a bathroom just before 1 p.m. with a backpack and reemerged a minute or two later without the bag "but with a gun in hand," Oakland County Assistant Prosecuting Attorney Marc Keast said during Wednesday's arraignment.
He allegedly walked down the hallway, aiming inside classrooms and at nearby students, according to Keast.
He never went into a classroom and was apprehended in a hallway, Bouchard said.
Thirty spent shell casings have been recovered, the sheriff said. The suspect had 18 live rounds left, he said.
The suspect's father purchased the weapon on Black Friday and officials are looking into how the family stored its guns and how much access the teen had to them, according to a source briefed on the investigation. The suspect had apparently used the gun prior to the school shooting, the source said.
McDonald said prosecutors are considering charges against both of the suspect's parents.
The first three students killed in the Tuesday shooting were Madisyn Baldwin, 17, Tate Myre, 16, and Hana St. Juliana, 14.
Four of the seven injured victims remained in the hospital on Wednesday, the sheriff said. Among those in the hospital is a 17-year-old girl who is in critical condition with a gunshot wound to the chest, he said.
In a statement released Wednesday evening, the Oxford Community Schools District said the community "has suffered a loss that is immeasurable."
The district said it is "fully participating" in the sheriff's office's investigation and warned against "significant speculation and misinformation" in the wake of the attack.
"The verified facts have been and will continue to be shared by law enforcement," the district said.
ABC News' Will Gretsky contributed to this report.