Metro yesterday blamed a bus driver and a faulty mechanical device for the Dec. 6 death of a 2-year-old Brentwood boy who was run over by the rear wheels of a bus after he and his mother left through the rear door.

"The primary cause of the accident was the failure of the bus operator to observe that the alighting passengers were not clear of the doors," according to a Metro report released yesterday.

The transit agency said the victim, TaVon Gamble, got off the bus with his mother, Deborah C. Woodruff, and his brother, Timothy Gamble, 4, at a bus stop on 38th Avenue just south of Allison Street in Hyattsville. Woodruff was standing in the rear stairwell and TaVon Gamble was off the bus when it lurched forward, said the Metro report.

Metro could not determine how the boy fell under the wheels, safety director Roger W. Wood Jr. said at a meeting of the Metro board's safety committee.

An "interlock" device in the Grumman Flxible bus should have prevented it from moving forward while the rear doors were open, officials said.

The device, which had been inspected two days before the accident, did not work because of a loose microswitch, an electronic component that signals whether the doors are closed or open, the report said. Because the switch was mispositioned, it did not signal that the doors were open, Wood said.

Although the interlock problem contributed to the accident, Metro officials said, they emphasized that the device serves as a backup and that the driver is responsible for making sure that the doors are closed before the bus moves.

"The operator should have had his foot on the brake pedal when stopped, rather than relying on the traction interlock," the report said.

The bus driver, Philip S. Williams of the District, was suspended for 10 days, retrained and allowed to resume his duties last week.

Metro investigators could not determine why the interlock microswitch was loose and misaligned, but they do not believe that it was a maintenance problem, Wood said. "It is inherent in the design that it can go out of adjustment," he said.

Out of Metro's fleet of 1,563 buses, more than 600 have the same interlock, said Lloyd Gimple, acting general manager for bus service. He said most of those buses are due to be replaced in the next few years and that the new buses have a different interlock system.

The interlocks are inspected every two months or every 5,000 miles, a spokeswoman said.

TaVon Gamble's family has filed a wrongful death lawsuit against Metro and Williams, seeking $6.5 million in damages.

Reporting on other accidents, Metro officials said:An exposed electrical wire was the source of the current that electrified the metal plate blamed for the death of a 12-year-old Northeast Washington girl June 4. The plate covered a box of electrical wires that powered the lights and gates of the Addison Road subway station parking lot in Prince George's County.

The transit agency said it does not know how the wire's insulation became worn, or how the wire came into contact with the plate. Metro has not determined whether suicide or an accident caused the death of Timothy Sexton, 76, of New York City, who fell into the path of a train May 15 at the Cleveland Park station in Northwest Washington.