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 victims included a 16-year-old boy who died in a deputy’s patrol car on the way to a hospital. Seven 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. Authorities said they were searching the suspect’s cellphone, school video footage and social media posts.

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

Deputies rushed to the school around lunch time and arrested the suspect in a hallway within minutes. 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 did not 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 identified as 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.

(AP)