OXFORD TOWNSHIP, Mich. — A fourth student, a 17-year-old boy, died Wednesday from wounds he suffered when a sophomore opened fire at a Michigan high school a day earlier, authorities said.

The other dead included a 16-year-old boy who died in a deputy’s patrol car on the way to a hospital. Eight people were wounded, some critically, including a 14-year-old girl who was placed on a ventilator after surgery.

Investigators were still trying to determine a motive for the shooting Tuesday at Oxford High School, located in a community of about 22,000 people roughly 30 miles (48 kilometers) north of Detroit, Oakland County Sheriff Michael Bouchard said.

“The person that’s got the most insight and the motive is not talking,” he said at a news conference late Tuesday.

Deputies rushed to the school around lunch time as more than 100 calls flooded 911 dispatchers with reports of a shooter. They arrested the student in a hallway within minutes of their arrival. He put his hands in the air as deputies approached, Bouchard said.

The boy’s father on Friday bought the 9 mm Sig Sauer used in the shooting, Bouchard said. He didn’t know why the man bought the semiautomatic handgun, which his son had been posting pictures of and practicing shooting, Bouchard said.

Authorities did not immediately release the boy’s name.

The four students who were killed were 16-year-old Tate Myre, 14-year-old Hana St. Juliana, 17-year-old Madisyn Baldwin and Justin Shilling, who died Wednesday.

Bouchard said Myre died in a patrol car as a deputy tried to get him to an emergency room.

A teacher who received a graze wound to the shoulder left the hospital, but seven students ranging in age from 14 to 17 remained hospitalized through the night with gunshot wounds, he said.

The gun the boy was carrying had seven more rounds of ammo in it when he surrendered, Bouchard said.

Undersheriff Mike McCabe said the student’s parents advised their son not to talk to investigators. Police must seek permission from a juvenile’s parents or guardian to speak with them, he added.

Oakland County Prosecutor Karen McDonald said in a statement that her office expects to issue charges quickly and that an update would be given Wednesday.

Authorities were made aware of posts on social media that said there had been threats of a shooting at the roughly 1,700-student school, but Bouchard said they didn’t know about the rumors until after the attack.

He stressed how crucial it is for tips of the kind to be sent to authorities, while also cautioning against spreading social media rumors before a full investigation.

McCabe also downplayed the significance of a situation in early November when a deer head was thrown off the school roof, which he said was “absolutely unrelated” to the shooting. The vandalism prompted school administrators to post two letters to parents on the school’s website, saying they were responding to rumors of a threat against the school but had found none.

Bouchard said the student in custody had no previous run-ins with his department and he wasn’t aware of any disciplinary history at school.

“That’s part of our investigation to determine what happened prior to this event and if some signs were missed how were they missed and why,” he said.

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

The district said in a statement that all of its schools would be closed for the rest of the week.

Isabel Flores, a 15-year-old ninth grader, 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, she said.

Authorities said they were searching the suspect’s cellphone, school video footage and social media posts for any evidence of a possible motive.

School administrators had posted two letters to parents on the school’s website in November, 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 deer 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 during the early morning hours. Without specifically referencing that incident, a second post on Nov. 12 assured “there has been no threat to our building nor our students.”

Both the sheriff and undersheriff emphasized that Tuesday’s shooting was unrelated to the deer head or any earlier investigation by their office.

“That was a different incident, different student,” McCabe said.

A concerned parent, Robin Redding, said her son, Treshan Bryant, is a 12th grader at the school but stayed home Tuesday. Redding said her son had heard threats that there could be a shooting.

“This couldn’t be just random,” 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 a vigil Tuesday night at LakePoint Community Church, Leeann Dersa choked back tears as she hugged friends and neighbors. Dersa has lived nearly all of her 73 years in Oxford. Her grandchildren attended the high school.

“Scared us all something terrible. It’s awful,” Dersa said of the shooting.

Pastor Jesse Holt said news of the shooting flooded in to him and his wife, including texts from some of the 20 to 25 students who are among the 400-member congregation.

“Some were very scared, hiding under their desks and texting us, ‘We’re safe, we’re OK. We heard gunshots, but we’re OK.’ They were trying to calm us, at least that’s how it felt,” he said.