The National Transportation Safety Board said Friday in a preliminary report on the cause of a collision last month between an Air Canada regional jet and an emergency vehicle at New York's LaGuardia Airport that air traffic controllers having to deal with heavy runway traffic was among the contributing factors. 

The report also found that several emergency vehicles were operating on the airport runways, responding to a plane that had reported smoke in the cabin. The vehicles didn't contain ADS-B transponders, which provide air traffic control with location and altitude information for aircraft in the air. 

The March 22 accident killed both pilots and sent 39 passengers to hospitals.

At airports equipped with systems to integrate the transponder signals for aircraft and vehicles on the ground, the transponders can also show locations on the ground and provide alerts about possible conflicts. 

As the Air Canada jet was landing on the runway, the ground controller initially cleared the vehicle to cross the runway. The controller changed his mind and told the vehicle to "stop, stop, stop," but the firefighters failed to understand that the instructions were meant for them. 

The report also said two controllers were on duty on the night of the crash and that the airport traffic at the time was heavier than usual and that they were dealing with multiple situations.

The NTSB said its final report will be released in two years.