The three killed at Oxford High School are believed to be students, Oakland County Undersheriff Michael G. McCabe said at a news conference. One of those shot is believed to be a teacher.
The suspect was identified as a 15-year-old sophomore boy, McCabe said.
Calls for an active shooter came in around 12:51 p.m. ET.
“We received over 100 911 calls to our dispatch center,” McCabe said.
The suspect was apprehended without incident within five minutes of officers arriving, McCabe said.
A handgun was recovered from the suspect, McCabe said. Authorities recovered multiple shell casings in the school and believe around 15-to-20 shots were fired. There is no indication the suspect was wearing body armor, McCabe said.
“At this point in time, we believe he acted alone,” McCabe said. He said that authorities are aware of how the suspect allegedly got the gun into the school, but declined to divulge these details.
The 15-year-old did not resist arrest, has invoked his right not to speak and is not telling law enforcement officers anything right now, McCabe said.
Officers were searching the school for possible additional victims.
Oxford Community School Superintendent Tim Throne said during the news conference the shooting left him shocked. “It’s devastating,” he said.
Throne declined to say why there were no metal detectors in the school, but did say — to his knowledge — metal detectors never came up in conversation prior to the shooting.
All evacuated students were relocated to a nearby store for reunification with relatives. About 25 agencies and close to 60 ambulances responded, according to John Lyman, public information officer for the Rochester Hills Fire Department.
Oxford Community School officials released a statement confirming the shooting and saying the “Oakland County Sheriff’s Department has secured the scene.”
McCabe said that students at the high school were being interviewed by law enforcement.
Oxford is about 45 miles north of Detroit.
CNN’s Alex Harring contributed to this report.