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(CNN) — Three people are dead and six were injured in a shooting Tuesday at Oxford High School in Oxford, Michigan, authorities said.

The three killed 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, who is a sophomore at the high school, 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 police arrival, McCabe said. A handgun was recovered at the scene from the suspect, McCabe said.

“At this point in time, we believe he acted alone,” McCabe said.

Officers were searching the school for possible additional victims.

All evacuated students were relocated to a nearby store for reunification with relatives.

Oxford Community School officials released a statement confirming the shooting and saying the “Oakland County Sheriff’s Department has secured the scene.”

“Oxford High School students and staff are systematically being evacuated to the Meijer Garden Center and may be picked up there. Any students with their own transportation have been allowed to leave. All other district schools are in lockdown for safety purposes and are in no danger,” read the statement, which was obtained by CNN affiliate WDIV.

Oxford is about 45 miles north of Detroit.