It was late at night and traffic was light, but unfortunately not light enough. A car traveling through the Big Dig Tunnel in Boston was in the wrong place at the wrong time when four concrete slabs weighing several tons fell from the ceiling, killing a woman.

Her husband escaped serious injury. He saw the panels starting to fall and slammed on his car’s brakes, but could not stop in time. Investigators found many other areas of the tunnel where bolts and plates holding the slabs to the ceiling had become loosened.

The attorney general’s office said the collapse site is being treated as a crime scene and negligent homicide charges potentially could result.