[Acey]

The "how did this happen" is that the controller made a mistake and cleared the fire truck into the path of the landing aircraft. He tried to correct the mistake but it was too late.

I've worked on this exact plane, CRJ have very bright landing lights that are tough to mistake as anything other than a plane, but there are lots of lights on an airfield and it could be hard to discern in the dark and rain. "Look both ways before crossing the street" still applies after a controller has given you clearance to cross for this exact reason. My condolences to the families of the dead but also to the tower controller who now has to deal with this mistake for the rest of his life; a lot of blame here will be rightfully directed at an increasingly broken US air traffic control system but ultimately one man made a catastrophic error with horrendous consequences.

It's a small and quite connected commercial aviation industry in Canada, many will have either worked with one of the crew lost or know somebody that did.