The student accused of killing four people in a Georgia high school shooting rode the school bus that morning with a semiautomatic assault rifle concealed in his backpack, investigators confirmed Thursday.

Colt Gray then left his second-period classroom after getting permission to go to the front office at Apalachee High School but hid from teachers in a bathroom before emerging to begin the assault, according to the Georgia Bureau of Investigation and Barrow County Sheriff Jud Smith.

The new details fill in some key questions about how the 14-year-old got a gun that could not be folded down to the school in Winder, northeast of Atlanta, before a shooting that killed two students and two teachers and injured nine others. 

The teen’s grandfather told CBS News that the morning of the shooting, his mother, Marcee Gray, received a cryptic message from her son. 

She called the school in a panic on the morning of Sept. 4 after getting the message, which said “I’m sorry,” and said Colin Gray, the boy’s father, received similar text messages that morning.

Marcee Gray said school officials told her on that call that they were already worried about her son’s behavior. He had enrolled at the school after it had already begun Aug. 1, Smith has said.

“The counselor said, ‘Well, I want to let you know that earlier this morning, one of Colt’s teachers has sent me an email that said Colt had been making references to school shootings,” Gray said in an interview with ABC News.

Marcee Gray and other relatives on her side of the family have said they had sought the school’s assistance the week before the shooting to get psychiatric treatment for the teen.

“I wanted Colt to be admitted to an impatient treatment,” Gray told ABC News. “Colt was on board with it.”

A school employee went to look for Colt Gray the morning of the shooting, but confused him with a fellow student with the same last name and similar first name, police and a student said. By that time, Gray had had left his second-period algebra class, going to the bathroom instead of the front office, investigators said.

The accused shooter is charged as an adult with four counts of murder, and District Attorney Brad Smith has said more charges are likely to be filed against him in connection with the wounded. 

Authorities have also charged his father, Colin Gray, alleging that he gave his son access to the gun when he knew or should have known that the teen was a danger to himself and others. He was charged with two counts of second-degree murder, four counts of involuntary manslaughter and eight counts of cruelty to children

Last year, Colt Gray and his father were questioned by FBI agents about reports of online posts threatening a school shooting. In bodycam footage released earlier this week, the teen can be seen denying writing the posts. Investigators say they didn’t have enough evidence for an arrest at the time.

Seven months later, Colin Gray allegedly gave his son a rifle for Christmas. The gun was purchased by the teen’s father as a gift, according to four federal law enforcement sources close to the investigation.

The 13,000 students at Barrow County’s other schools returned to class Tuesday. Officials have not announced a restart date for the 1,900 students who attend Apalachee.

Here’s a timeline of what happened on the day of the attack, based on statements by authorities and reporting by The Associated Press and other news media:

8:15 a.m. – First period begins. Officials have not said what class Colt Gray was scheduled for, or if he attended. Officials said Colt Gray rode the school bus to Apalachee High School carrying a semiautomatic assault rifle hidden in his backpack.

9:38 a.m. – First period ends. Students have seven minutes to change to their next class.

9:45 a.m. – Second period begins. Student Lyela Sayarath said she briefly saw Gray in the algebra class where the two sat next to each other. Investigators say Gray left the classroom asking to go speak to someone in the front office, but instead took his backpack with the gun inside and hid in a bathroom.

9:50 a.m. – Marcee Gray, Colt’s mother, calls the high school from 200 miles away in Fitzgerald, Georgia, to warn that her son was having an “extreme emergency” after getting a text from Colt saying “I’m sorry.” Marcee Gray said a counselor said they had received an email earlier that morning from one of Colt Gray’s teachers saying he had been talking about school shootings. Gray said she urged them to find her son to check on him. Call logs show the call lasted until 10 a.m.

9:45 a.m. to 10:20 a.m. – An administrator comes to the algebra classroom looking for a student with the same last name and a similar first name to Colt Gray, Sayarath and Barrow County Sheriff Jud Smith said. When the other student returns, he tells Sayarath that the administrator was actually seeking Colt Gray. In the meantime, the teacher is called on the intercom, Sayarath said.

About 10:20 a.m. – Colt Gray approaches the door of the algebra classroom. As the intercom buzzes again, the teacher responds, “Oh, he’s here,” seeing Gray outside the classroom door, Sayarath said. When students go to open the door, which automatically locks from the inside when closed, Sayarath said they backed away. She said she saw Colt Gray turn away through the window of the door and then she said she heard 10 or 15 consecutive gunshots. People are shot in the hallway and inside at least one classroom, as others in the halls scramble for safety. According to some students, the three teachers who are shot are trying to protect students.

10:23 a.m. – After multiple employees press wireless panic buttons embedded in their employee badges, the school goes into lockdown and a massive law enforcement response begins. Students in other classrooms who hear the gunshots begin texting and calling their parents and others.

10:26 a.m. – The two school resource officers assigned to Apalachee High School approach Gray in the hall, according to GBI Director Chris Hosey. Gray immediately surrenders and is taken into custody.

About 11 a.m. – Law enforcement officers begin searching Colin and Colt Gray’s house east of Winder. At the school, officers go from classroom to classroom, first looking for more people with injuries or other shooters. Later, officers evacuate students to the football field as hundreds of parents rush to campus.

About 1 p.m. – The school begins releasing students to parents to take them home.