A 15-year-old sophomore opened fire at his Michigan high school on Tuesday, killing three students and wounding six other people, including a teacher, authorities said.

Oakland County Undersheriff Mike McCabe said at a news conference that he didn’t know what the assailant’s motives were for the attack at Oxford High School in Oxford Township, a community of about 22,000 people roughly 30 miles (48 kilometers) north of Detroit.

Officers responded at around 12-55 p.m. to a flood of 911 calls about an active shooter at the school, McCabe said. Authorities arrested the suspect at the school and recovered a semi-automatic handgun and several clips.

“Deputies confronted him, he had the weapon on him, they took him into custody,” McCabe said, adding that suspect wasn’t hurt when he was taken into custody and he refused to say how he got the gun into the school.

Authorities didn’t immediately release the names of the suspect or victims. About 1,700 students attend the school.

ALSO READ: Taliban kill, abduct dozens of ex-officers: Rights group

Tim Throne, the superintendent of Oxford Community Schools, said he didn’t know yet know the victims’ names or whether their families had been contacted.

“I’m shocked. It’s devastating,” the shaken superintendent told reporters.

The school was placed on lockdown after the attack, with some children sheltering in locked classrooms while officers searched the premises. They were later taken to a nearby Meijer grocery store to be picked up by their parents.

Isabel Flores told WJBK-TV that she and other students heard gunshots and saw another student bleeding from the face.

They then ran from the area through the rear of the school, said Flores, a 15-year-old ninth grader.

McCabe said investigators would be looking through social media posts for any evidence of a possible motive.

Robin Redding said her son, Treshan Bryant, is a 12th grader at the school but stayed home on Tuesday. She said he had heard threats of a shooting at the school.

“This couldn’t be just random,” she said.

Redding didn’t provide specifics about what her son had heard, but she expressed concern with school safety in general.

“Kids just, like they’re just mad at each other at this school,” she said.

Bryant said he texted several younger cousins in the morning and they said they didn’t want to go to school, and he got a bad feeling. He asked his mom if he could do his assignments online.

Bryant said he had heard vague threats “for a long time now” about plans for a shooting at the school.

“You’re not supposed to play about that,” he said of the threats. “This is real life.”

School administrators posted two letters to parents on the school’s website this month, saying they were responding to rumors of a threat against the school following a bizarre vandalism incident.

According to a Nov. 4 letter written by Principal Steve Wolf, someone threw a dear head into a courtyard from the school’s roof, painted several windows on the roof with red acrylic paint and used the same paint on concrete near the school building.

Without specifically referencing that incident, a second post on Nov. 12 assured “there has been no threat to our building nor our students.”

“We are aware of the numerous rumors that have been circulating throughout our building this week. We understand that has created some concern for students and parents,” the administrators wrote. “Please know that we have reviewed every concern shared with us and investigated all information provided. Some rumors have evolved from an incident last week, while others do not appear to have any connection. Student interpretations of social media posts and false information have exacerbated the overall concern.”

Michigan’s governor, Gretchen Whitmer, was one of several elected officials who expressed condolences to the victims.

“Gun violence is a public health crisis that claims lives every day. We have the tools to reduce gun violence in Michigan. This is a time for us to come together and help our children feel safe at school,” Whitmer said in a statement.

ALSO READ: New Chinese surveillance system targets journalists, foreign students