BART investigators are focusing their probe of the Feb. 21 train derailment in Concord on the four cars that jumped the tracks after they cleared the part-time train operator from any wrongdoing, a manager told the board late Thursday.

Paul Oversier, assistant general manager of operations, gave the most detailed explanation of the accident to date, saying inspectors have yet to find a "smoking gun."

"On these kind of incidents we prefer to have a smoking gun because then we know what to fix," Oversier told the BART board of directors at the end of Thursday's long meeting.

At 6:25 p.m. on Feb. 21, during the evening commute, the first four cars of a 10-car, out-of-service train jumped the tracks, leaving the lead car dangling over the edge of the elevated rails. The train had completed its passenger service at the North Concord station, then headed south to the Concord maintenance yard with just the operator on board.

After leaving the Concord station, the train crossed a set of switches that transfers trains from one track to another. At that point the front set of wheels on the second car "climbed the rail" as the train was traveling 24.5 mph, Oversier said.

The speed limit for that section is 27 mph. The train skidded about 24 feet past the switch point, Oversier said.

The train was under computer control at the time of the incident with a part-time train operator, who has worked at BART for 16 months, in the cab.

"That eliminates the train operator as being a possible contributing factor in the accident," Oversier said, adding that the computer operated properly.

Eleven minutes before the derailment, another eight-car train traveled the same path without incident, Oversier said.

Investigators determined that another slow-speed derailment on March 13, 2011 on the same section of tracks was caused by uneven wear on a track section and flawed maintenance on a train wheel that slipped off the rails.

The agency added wheel wear standards after that accident, Oversier said.

Investigators have cleared the track as the cause of the latest derailment. The rails were last inspected Nov. 30 and again following the accident, both times meeting BART's standards, Oversier said.

The California Public Utilities Commission also cleared the track and will continue to aid in the probe, he said.

Oversier said the cars will be examined by experts from the Transportation Technology Center in Colorado, a research nonprofit that will offer technical expertise.

Initial reports on the cars show no obvious issues. The wheels were cut more than six months ago and measured within standards, and the maintenance record appears normal.

The train's black boxes indicated an axle locked up, which would be a big deal, Oversier said. However, investigators believe that happened after the derailment.

"We're still in the very early stages of what I think will be a very complicated investigation," Oversier said, adding the 2011 investigation took five months.

The derailment could prove costly as the first two cars are "not repairable," while the second two will cost a "considerable amount of money" to fix.

In an effort to make it up to commuters inconvenienced by the derailment and other equipment malfunctions this week, BART reps handed out $20 tickets and apologies at the North Concord station Friday night.

Contact Matthias Gafni at 925-952-5026. Follow him at Twitter.com/mgafni.