SEATTLE -- Seattle monorail officials plan to bring in a crane Monday to pull apart the two trains that are wedged together on a curve.

Investigators are looking at three possible causes for the crash -- operator error, signal malfunction or communications problems.

The trains came to a screeching halt Saturday night in a shower of sparks and breaking glass. Two people were taken to hospital with minor injuries after firefighters rescued 84 passengers by ladders.

It happened at the spot where tracks were "pinched" in 1988 to fit with the new Westlake Center.

To prevent collisions departing drivers are supposed to radio the operations center and arriving drivers have to watch for a green arrow signal that the track is clear. That worked for 17 years.

Stuart Rolfe of Seattle Monorail Services, which operates the system for the Seattle Center, called the collision a "breakdown in protocol."