View of the school grounds as emergency personnel respond to deadly shooting in Oxford, Michigan. Reuters

A 15-year-old sophomore opened fire at his Michigan high school on Tuesday, killing three students, including a 16-year-old boy who died in a deputy’s patrol car on the way to a hospital, authorities said. Eight other people were wounded, some critically.

Oakland County Sheriff Michael Bouchard told a news briefing hours after the rampage at Oxford High School that investigators were at a loss to explain what might have precipitated "an unspeakable and unforgivable" act of violence.

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The suspect, disarmed and taken into custody by sheriff's deputies minutes after the shooting began, has declined to speak with investigators after his parents retained a lawyer and denied authorities permission to interview their son, Bouchard said.

"The person who's got the most insight on motive is not talking," the sheriff told reporters.

Oakland County Undersheriff Mike McCabe speaks at a news conference in Oxford, Michigan. AP

The suspect's father had bought the 9mm Sig Sauer used in the shooting on Friday, Bouchard said, adding that he did not know why the man bought the gun. Bouchard said the suspect had practiced shooting with the gun and "posted pictures of the target and the weapon."

Undersheriff Mike McCabe said earlier that authorities were aware of allegations circulating on social media that there had been threats of a shooting at the roughly 1,700-student school before Tuesday's attack, but he cautioned against believing that narrative until investigators can look into it.

McCabe said the suspect's parents visited their son where he's being held and advised him not to talk to investigators, as is his right. Police must seek permission from a juvenile suspect's parents or guardian to speak with them, he added.

President Joe Biden, before delivering remarks at a community college in Rosemount, Minnesota, said: "As we learn the full details, my heart goes out to the families enduring the unimaginable grief of losing a loved one."

A police road block restricts access to Oxford High School following a shooting in Oxford, Michigan. AFP

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.

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.