Investigators are still trying to determine what caused two planes to crash in midair Saturday morning in Boulder County, looking into the possibility that one of the pilots lost sight of the other plane while making a turn.

The collision occurred near Niwot Road about 2 miles south of Longmont, killing both pilots and a passenger, Boulder County sheriff’s deputies said.

As of Sunday afternoon, the coroner had yet to release the names of the people killed in the crash, which involved a Cessna 172 and a Sonex Xenos.

Neither plane had a collision avoidance system or a black box, investigators said.

In the next two weeks, the National Transportation Safety Board is expected to release its preliminary findings on what caused the crash. The agency will release a more detailed report over the next 18 months.

Investigators said they believe that the pilot of the Cessna might have lost track of the other plane.

“The Cessna made a 360-degree left turn and then began a right turn toward the east and about that time the two flight tracks of the aircraft merged, and then both aircraft descended rapidly until impacting a rural area,” said Mike Folkerts of the National Transportation Safety Board.

Saturday’s crash occurred two months after a plane crashed near Lefthand Canyon Drive in rural Boulder County, killing all four people aboard the plane.

The victims were Steven Chase, 32, Sandra Kirby, 48, Amanda Kirby, 13, and Ian Kirby, 17. The National Transportation Safety Board and Federal Aviation Administration are still investigating the crash.