LAGUARDIA AIRPORT — A Port Authority fire truck crashed into a landing passenger airplane on a runway at LaGuardia Airport, killing two pilots and sending more than 40 people to the hospital. The runway safety system did not issue an alert before the collision.

Investigators continue to probe for answers in the collision, focusing on why the ASDE-X safety system failed to generate a warning. LaGuardia Airport uses ASDE-X to track surface movements of aircraft and vehicles. The system did not generate an alert due to the close proximity of vehicles merging and unmerging near the runway, which prevented it from creating a track with high confidence.

The fire truck involved in the collision did not have a transponder and was responding to a separate emergency involving a United Airlines flight at the time. According to the cockpit voice recorder, the truck was cleared to cross the runway 20 seconds before the collision. Nine seconds before impact, the air traffic control tower ordered the truck to stop, followed by a second stop order four seconds before the collision.

"ASDE-X did not generate an alert due to the close proximity of vehicles merging and unmerging near the runway, resulting in the inability to create a track with high confidence," said Jennifer Homendy, chairwoman of the National Transportation Safety Board. "I do not know, we have no indication that was a factor here, but it is a shift that we have been focused on in past investigations."

Two controllers were on duty in the air traffic control tower that night: a local controller and the controller in charge, who was also performing the duties of the clearance delivery controller during the shift. Investigators have not yet determined who was performing the duties of the ground controller, who manages all aircraft and vehicle movement on taxiways, at the time of the collision. "The truck having a transponder very well could have provided that alert that the controller could have used," said Jeff Guzzetti, an aviation analyst.

Photographs of the wreckage showed that the plane's nose was torn off, with mangled pieces dangling toward the ground. The two pilots on the Canada Air Express plane were killed in the collision.

Stephen Abraham, a retired air traffic controller with 28 years in the industry, said it could be close to a year before investigators have a full picture of what happened. "If you put three people on the midnight shift, it means you have less people during the day and at night when it's busier. There's just not enough people," Abraham said. "You can't invent or make people." Abraham said he understands the National Transportation Safety Board's concern but stressed that the ongoing staffing shortage of controllers remains a problem.