Two small planes collided in a flames over northern Colorado and killed all three people aboard, but a glider under tow by one aircraft cut loose and flew through the fireball to safety, officials and witnesses said.

Three people were aboard the glider that managed to disconnect from a Piper Pawnee as a Cirrus SR20 clipped the tow line an instant before the two planes collided, Boulder County sheriff's office spokesman Rick Brough said.

Both aircraft plummeted toward the ground but the glider landed safely with no injuries to any on board.

"We understand the glider went through a fireball after the impact," NTSB field investigator Jennifer Rodi.

The crash occurred about 1:30 p.m. Saturday near the Boulder Municipal Airport over a suburban area dotted with homes and businesses, but no one on the ground was injured.

Several people witnessed the accident, including two who described people falling from a plane.

"We heard a loud bang and looked up in the air and we saw what looked like a glider and big, black smoke right next to it," Paul Aiken told Boulder's Daily Camera. "It looked like fireworks, the explosion."

The Denver Post quoted Sue Patton, 53, as saying she saw two people plunge out of a plane.

"The plane was burning really strong," she said. "They really didn't have a choice."

The pilot of the glider was Ruben Bakker, according to his mother-in-law Deborah Tjarks, who spoke to The Associated Press. She said he saw the collision about to happen and released the glider and banked but still flew through the flames. Bakker did not immediately return a call for comment.

Brough said the Piper Pawnee with just a pilot aboard belonged to Mile High Gliding Inc. and had just taken off from the Boulder airport with the glider in tow.

A woman who answered the phone at the glider company declined to comment.

The other plane, a single-engine, four-seat Cirrus SR20, was carrying two people.

Brough said the three people aboard the two planes were killed. He said there was initial concern that there could have been others aboard the Cirrus because of its number of seats but investigators were now "pretty certain" there were just the two aboard.

Gliders, or sailplanes, are lightweight aircraft that are often towed into the sky, then released to glide to the ground.

An amateur video shot at the scene also showed a plane on fire, floating to the ground trailing thick, black smoke and a parachute.

Brough said the parachute was designed to deploy if a plane was disabled and was attached to the plane's wreckage, not a pilot or passenger.

It wasn't immediately known how the Cirrus got close enough to the Piper Pawnee to clip its towline.

Brough said the identifications of the victims were being determined by the coroner's office.