(CNN) -- Two planes collided in midair Saturday in Boulder, Colorado, killing three people, authorities said.
One of the planes, a single-seat Piper Pawnee, was towing a glider. The pilot of the Piper died in the collision, said Rick Brough of the Boulder County Sheriff's Office.
But the pilot of the glider, which was carrying two passengers, detached from the plane and landed safely, Brough said.
Two people on the second plane, a Cirrus SR 20, also died, Brough said.
Authorities began receiving reports of a plane crash around 1:30 p.m. MT, Brough said, adding that people had seen flames in the air in an area less than five miles from the Boulder Municipal Airport.
The collision was described as "immediate disintegration and explosion of both planes," said Jennifer Rodi, an air safety investigator with the National Transportation Safety Board.
Rodi called the glider's occupants "very lucky" during a news conference Sunday and praised the glider's pilot.
"The glider was attached to the Piper Pawnee when the pilot observed the Cirrus aircraft out of the corner of his eye," she said. "He immediately noted they were on a collision course and elected to separate the tow strap from the Piper Pawnee."
"He did his job that he signed up for as a commercial pilot," Rodi said.
Rodi said the Cirrus was traveling south and the Pawnee was traveling west at the time of the collision.
The debris field was spread out over a couple of miles, Brough said.
"I was in my house and I heard a loud bang," iReporter Zac Mitchell, of Boulder, told CNN. "I looked out my window and saw a giant fireball in the sky."
When he went outside, he said he saw smoke and two objects, including a parachute, falling from the sky.
Cirrus aircraft come equipped with an airframe parachute system that deploys in the event of an in-air emergency. It's designed to safely lower the plane to the ground.
The NTSB and the Federal Aviation Administration are involved in investigating the incident, Brough said.
CNN's Leslie Tripp, John Branch and Nick Valencia contributed to this report.