Eight people were injured, some extremely serious, including a 14-year-old girl placed on a respirator after operating on her.
A fourth student, 17, died Wednesday of injuries sustained when a sophomore opened fire at a Michigan high school the day before, authorities said.
The other fatalities include a 16-year-old boy who died on a police patrol taking him to a hospital. Eight people were injured, some extremely serious, including a 14-year-old girl placed on a respirator after operating on her.
Investigators are still trying to uncover the motive for the shooting Tuesday at Oxford High School, located in a town of 22,000 about 30 miles north of Detroit, Oakland County Sheriff Miguel Bouchard said.
" The person who knows the reason best does not speak, " he told a news conference Tuesday night.
Police officers flocked to the school at lunchtime when a hundred calls to 911 reported they heard gunshots. The student was arrested minutes after the rapture. He raised his hands as officers approached, Bouchard said.
The commissioner said the arrested boy's father bought the 9mm Sig Sauer pistol used in the massacre on Friday. He said he did not know why the man believed the semi-automatic pistol. The son had posted photos of the gun and had practiced shooting it.
Authorities did not release the boy's name.
The four students killed were Tate Myre, 16; Hana St. Juliana, 14; Madisyn Baldwin, 17; and Justin Shilling, who died Wednesday.
Myre died on patrol when an agent was taking him to an emergency room.
A teacher who received a superficial wound to the shoulder left the hospital. At the same time, seven students between 14 and 17 years old remained hospitalized with injuries of varying severity.
The gun the boy was carrying still had seven rounds in the magazine when he surrendered, Bouchard said.
Assistant Commissioner Mike McCabe said the parents advised the boy not to speak to investigators. Police can only question a minor with parental permission, he added.
In a statement, Oakland County Prosecutor Karen McDonald said that her office plans to file charges quickly and that more information will be released in the next few hours.
Bouchard stated that the boy had not had any encounters with the local police or, to his knowledge, disciplinary problems at the school.