Two small airplanes collided in midair near Denver on Saturday, killing three people, authorities said.

It wasn’t known what might have caused the crash between the four-seater Cessna 172 and the Sonex Xenos, a light, aluminum, homebuilt aircraft that can seat two.

Nearby residents who heard the crash and rushed outside told CBS affiliate 9News in Denver they were surprised the planes could have struck each other on such a clear morning.

One of the planes crashed in a field, the other in a stand of trees, near Vance Brand Airport in Longmont, Colorado, about 30 miles (50 kilometers) north of Denver. Two people were found dead in one wreckage, and one person in the other, the Boulder County Sheriff’s Office said.

The Federal Aviation Administration and the National Transportation Safety Board are investigating. The NTSB said a preliminary report would be released in about 15 days.

The collision happened just before 9 a.m. Saturday. The identities of the victims were not immediately released.