Federal investigators say an airplane pilot never acknowledged a controller's warning before his plane collided with another.
The Piper Cheyenne and a Cessna 172 both crashed to the ground Jan. 24 after colliding in midair.
The crash killed all five people on the planes and rained debris over a 24-block area. The Cessna smashed into a house and ruptured a natural gas line.
That triggered an explosion a few minutes later.
A preliminary report from the National Transportation Safety Board also indicates an apparent problem with an altitude transponder on the Piper.
The NTSB says the pilot reported his airplane was transmitting altitude information to air traffic control even after a controller said the signal wasn't registering.
The report says controllers also warned the Piper Cheyenne pilot that the Cessna was directly ahead, but he never responded.
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