CHICAGO (AP) — Accident investigators say heat likely caused a coal train derailment last July that sent a railway bridge crashing down onto a road north of Chicago, killing a couple who were driving underneath.

The Federal Railroad Administration says it is likely that temperatures of 103 degrees heated the track and caused it to buckle a few feet before the bridge near the suburb of Northbrook.

The fully loaded coal train weighing more than 19,000 tons when it derailed, sending 32 freight cars off the track. Nearly all of them piled up in a spectacular mound directly on top of the overpass, causing it to collapse.

Zorine and Burton Lindner were traveling under the bridge at the time and were killed.

Their sons are suing Union Pacific Corp., alleging negligence.