Port Authority is focusing on a metal pin that keeps the locking mechanism engaged on the rear doors of its buses as the culprit in Friday's accident that led to a woman being injured after she fell from a bus's back doorway.
The locking mechanism "failed" on the bus, but it is unclear why, the transit agency said Tuesday.
"We know what," Port Authority spokesman Jim Ritchie said. "We're trying to find out why that is."
The bus, on the 22 McCoy route, was taken out of service and inspected.
The agency checked the fleet's approximately 700 buses and did not find "other similar problems," Port Authority said in a statement.
Inspectors did find two bus doorways -- there are two rear doors on buses that come together and are supposed to remain shut while the bus is in motion -- that were not properly aligned "but did not pose the same risk to riders."
"We did find those issues and corrected them," Mr. Ritchie said. "Now we know that every vehicle is safe on the street."
The accident occurred Downtown about 6 p.m. Friday as the outbound bus turned the corner from Stanwix Street to Fort Duquesne Boulevard.
Mr. Ritchie said the passenger was standing at the top of the steps by the rear doors and fell against them as the bus was turning.
The woman fell into the street and was taken to UPMC Mercy, where she remains in the intensive care unit, Mr. Ritchie said. Port Authority has not identified her.
Bus drivers control the front and back doors with a lever.
When the rear doors are closed, a metal pin on a spring mechanism engages to keep the doors shut.
"In this case, it's not doing that," Mr. Ritchie said.
Asked whether the agency had ruled out driver error accounting for the doors opening, Mr. Ritchie said, "It appears to us that the cause has to do with the mechanism itself and not some outside intervention with the doorway or the lock."
The doors are made by Gillig LLC, which also makes the bus. Port Authority has not asked the company to become involved in its investigation.
Port Authority inspected the bus six months ago as part of a required state inspection. Inspecting the rear doors and the locking mechanism is part of the process, Mr. Ritchie said.
Jonathan D. Silver: firstname.lastname@example.org, 412-263-1962 or on Twitter @jsilverpg.