DENVER, Colo. -- Cracked windshields on 14 planes at Denver International Airport were caused by "foreign object debris," air safety investigators said Tuesday.

Microscopic analysis of the 21 front and side windshields cracked during a storm revealed fine particles causing pitting that in turn caused cracking, National Transportation Safety Board investigator Jennifer Kaiser said. Only the outer layer of the triple-layer windshields cracked and none of the planes declared emergencies.

Wind gusts at the airport reached 48 mph, said Kyle Fredin of the National Weather Service during the three hours Feb. 16 when cracks showed up on planes from SkyWest, Frontier, and Great Lakes airlines. Cold temperatures and snow were also reported.

"The only commonality across aircraft type, operator, location, time and phase of flight was the wind and weather," Kaiser said.

Some of the six planes developing windshield cracks during take off aborted the effort. One developed cracks after it landed, another two while taxiing to the terminal after landing, three as they were parked at the gate and one while being pushed back from the gate.

One plane developed the problem at altitude while the aircraft was at 19,000 feet, Kaiser said.

Investigators were unable to determine the precise nature of the debris.