Federal investigators say one of only two air traffic controllers on duty at LaGuardia Airport cleared a fire truck to cross a runway just 12 seconds before an Air Canada flight touched down, leaving little time to avoid the collision that killed both pilots. The National Transportation Safety Board is working to determine which of the airport’s layers of safety precautions failed and allowed the fire truck onto the runway Sunday night. Among the areas being explored are whether the common practice of two controllers on duty overnight is enough and why a runway warning system didn’t alert the possibility of a crash. The plane carrying more than 70 people slammed into the fire truck, killing two pilots.