At least four people were killed Saturday morning when two small planes collided in the air over Boulder County and then crashed.

The Federal Aviation Administration says a single-engine Cessna 172 collided with a second, unknown aircraft at about 9 a.m. near Vance Brand Airport in Longmont. The FAA said two people were aboard the Cessna but that it’s unclear how many people were aboard the other plane.

Mountain View Fire Rescue initially said three people were killed in the collision, which happened between North 95th Street and U.S. 287 off Niwot Road. The Boulder County Sheriff’s Office later said, however, that four people were killed in the crash.

“The FAA and the National Transportation Safety Board will investigate,” the FAA said in a news release. “The NTSB will be in charge of the investigation and will provide additional updates.”

The Sheriff’s Office said its detectives were also at the scene of the crash.

Authorities did not immediately release the identities of the people killed in the collision.

Midair collisions between aircraft are rare. The last one in Colorado happened in May 2021, when a Fairchild Swearingen Metroliner operated by Key Lime air collided with a Cirrus SR-22 while both aircraft were preparing to land at Centennial Airport.

The Cirrus, which was equipped with a parachute that deployed after the collision, crashed, but the two people aboard the plane were uninjured. The Metroliner landed safely but was heavily damaged.

This is a developing story that will be updated.