The Federal Aviation Administration (FAA) spokeswoman Laura Brown said, "One of the aircraft clipped the towline of the other plane that was pulling a glider."
According to officials, the victims were from both the planes.
The pilot of the glider, identified as Reuben Bakker, landed at Boulder Municipal Airport about 3 miles southeast of the crash site with his two passengers, a woman and her 11-year-old son, Denver Post reported.
"Investigators are trying to establish the identity of the deceased," Jennifer Rodi of the National Transportation Safety Board, lead investigator on the crash was quoted as saying.
Eyewitnesses said that the plane, which crashed after the accident looked like a fireball coming down from the sky.