A firefighter whose truck collided with an Air Canada jet last month on a runway at LaGuardia Airport in New York, killing both pilots, heard an air traffic controller warn 'stop, stop, stop' but didn’t know who it was for, federal investigators said Thursday. The National Transportation Safety Board stated in a preliminary report that a crash prevention system didn’t generate an alert in the control tower. At the time of the incident, the runway entrance lights were still on until moments before the collision. The pilot and co-pilot of Air Canada Express Flight 8646 tragically lost their lives while approximately 40 people, including the two individuals in the fire truck, were hospitalized. The accident raised concerns over airport safety systems as it was the first fatal crash at LaGuardia in 34 years.