Two squad cars speeding to an officer's call for backup collided at a downtown intersection early Monday, killing three officers and injuring a fourth in one of the worst traffic accidents in the department's history.

The two cars were responding to a backup call from officers trying to arrest five car theft suspects about 4 a.m. downtown, Officer Bill Frio said.They crashed at Fifth and Walls streets, one block north of the arrest site, Frio said.

The suspects had apparently been in custody before the crash and were not expected to be booked in connection with the fatal accident.

The impact of the crash pushed one of the patrol cars into a 20-foot-deep MetroRail excavation ditch, killing both officers in the car. Their names were withheld until relatives could be notified.

The other squad car slammed into a light standard, Frio said.

One occupant, identified as Officer Manny Gutierrez, 30, died of massive head injuries at County-USC Medical Center, a hospital spokeswoman said. The other, Officer Vincent Drake, 28, was treated for back injuries and listed in serious-but-stable condition.