A high school gunman who killed three students allegedly posed as a police officer in a bid to lure teens out of a locked classroom.
Three students were killed and eight other people were injured when a 15-year-old boy went on a rampage with a gun purchased by his father just days ago, say police.
Chilling footage shows terrified children hiding in a room with a teacher when a person says: "Sheriff's office. You can come out." But the teens rush to the back of the room, believing it to be the gunman.
Police said the suspect's father purchased the semi-automatic handgun four days before the mass shooting at Oxford High School in Oxford, Michigan.
Police identified the three victims as Hana St Juliana, 14, Tate Myre, 16, and Madisyn Baldwin, 17.
Two of the wounded were in surgery and six were in stable condition, said police.
The motive of the shooter was unknown, the Oakland County Sheriff's Office said.
Video posted on social media showed students huddled in a barricaded classroom while someone outside shouts through the door that it is safe to come out, identifying himself as being with the sheriff's office.
But the teacher responds: "We're not taking that risk."
The person then said "come to the door and look at my badge, bro."
The terrified students escaped through a window in the classroom and were met by uniformed police.
The shooting underscored how students must decide when to emerge from shelter once the gunfire stops.
Student Abbey Hodder told the Detroit Free Press she was in chemistry class when she heard the sound of glass breaking.
"My teacher kind of ran out and was scrambling," Hodder, 15, told the newspaper.
"The next thing I knew I saw he was pushing tables. It's part of school protocol to barricade, so we all knew, barricade, barricade down. And we all started pushing tables."
The suspect, a sophomore at the school, was believed to have acted alone and was arrested without resistance after firing 15 to 20 shots, Undersheriff Michael McCabe said.
"The whole thing lasted five minutes," McCabe said.
The suspect divulged nothing to police and demanded his right to a lawyer on being advised not to speak by his parents, McCabe said.
The latest in a long series of US school shootings will likely fuel debates about gun control and mental health care, with many states allowing easy access to firearms while mental health disorders frequently go untreated.
"This is a uniquely American problem that we need to address," said Michigan Governor Gretchen Whitmer, who rushed to the scene and appeared with McCabe before the media.
McCabe praised the school for its preparation for a shooting and an orderly evacuation.
"My heart goes out to the families enduring the unimaginable grief of losing a loved one," President Joe Biden told reporters while touring a Minnesota technical college.
Oakland County Prosecutor Karen McDonald promised swift justice, saying in a statement: "It is our intent to review it (the investigation) thoroughly and issue appropriate charges quickly. The suspect will remain in custody pending those charges."
