The crash occurred about 1:30 p.m. Saturday near the Boulder Municipal Airport when a southbound Cirrus SR20 collided with a westbound Piper Pawnee that was towing the glider, causing the "immediate disintegration and explosion of both airplanes," Rodi said.
The pilot of the glider was able to cut loose just before the collision and fly to safety.
The Boulder County Coroner's Office said Sunday that 25-year-old Alexander Howard Gilmer of Evergreen, Colo., was the pilot of the Piper Pawnee.
The coroner's office tentatively identified the pilot of the Cirrus as 58-year-old Robert Matthews of Boulder, and the passenger as 56-year-old Mark A. Matthews of Englewood, Colo.
Young Kim said he and his girlfriend were walking out of her condo Saturday when they heard a loud boom.
"We looked up in the sky. We saw a glider and right next to it what looked like a big black ball of fire," he said. "It looked at first like fireworks coming out of it."
His girlfriend, Barb Maiberger, said, "You're going, 'This can't be real.' But it was real, and I knew something was wrong."
Kim started running about a half-mile to the scene. "You could see a big smokestack coming out from the wreckage, and dozens of people running toward the scene hoping to rescue someone. As you got closer, you could actually smell the fumes from the jet fuel," Kim said.
"I was just hoping maybe somebody survived," he said.
Several witnesses have said they saw people plunging from the planes, but Rodi said it's hard to tell whether they saw people or airplane parts falling.
An amateur video shot at the scene showed a plane on fire, floating to the ground trailing thick, black smoke and a parachute. Sheriff's officials said the parachute was designed to deploy if a plane was disabled and was attached to the plane's wreckage, not a person.
The crash spread debris over a 1½ mile region of prairie. No one on the ground was hurt.
Three people were aboard the glider that managed to disconnect from the Piper Pawnee as the Cirrus clipped the tow line, just before the two planes collided, Boulder County sheriff's officials said.
The pilot of the glider was Ruben Bakker, his mother-in-law Deborah Tjarks said. She said he saw the collision about to happen and released the glider and banked but still flew through the flames. Bakker did not return a call for comment.
The single-engine Cirrus left the Boulder airport with two people on board around 12:45 p.m. Saturday and was lost on radar for about 10 minutes, Rodi said.
Sheriff's officials said the Piper Pawnee belonged to Mile High Gliding Inc. and had just taken off from the Boulder airport before the crash with the glider in tow.
The Cirrus had the capability to provide data from avionics, like a black box, but the avionics were destroyed in the crash and fire, Rodi said.
Investigators working as light snow fell Sunday planned to recover parts of the plane until dark, then start again Monday morning.
They will examine maintenance records, the pilots' flight records and look at paint transfers on the plane parts to help determine what speeds the planes were traveling.
Rodi wouldn't speculate on whether air congestion is a problem in the area.
In the airspace the planes were using, the pilots didn't have to communicate with air traffic control towers but could have communicated with each other. Rodi said investigators would talk with other pilots who might have heard the pilots of the two planes talking with each other. It wasn't immediately known how many other pilots were in the area at the time.