"a LaGuardia air traffic controller can be heard on a radio transmission first giving clearance to the fire truck to cross the runway, then trying to stop it, but it was too late to avoid the collision." ATC has never been a job I would want. And they've never been spread so thin and so overworked.
This is already distilled down into just a single question for me, which is "how and why did the ATC not realize there was a plane _actively_ landing on the same runway he just directed a firetruck to cross?"