7NEWS reporter Lindsay Watts said the plane was being flown to Broomfield for maintenance.
The single-engine plane went down next to Independence Road, near 57th Street. The wreckage is about 50 yards away from the runways at Boulder Municipal Airport.
Investigators said the plane took off from the airport at 7:40 a.m. and crashed minutes later.
"The preliminary investigation indicates that the plane struck the westbound lane of Independence Road propeller first, with the plane catching fire after impact," the Boulder County Sheriff's Office said in a news release.
A witness said he saw the plane as it went down the runway before takeoff and said it "was running horribly." He said soon he didn't see it take flight, but saw the fire after the crash.
"You could tell immediately there were no survivors," Jeff Slater said. "The flames engulfed everything--probably 30 foot flames."
The NTSB and FAA will investigate the crash. An NTSB official said the plane was a Mooney M20E.