Federal safety officials are investigating some of Montgomery County’s Ride On buses following fires that have destroyed five of them since 2009.

The buses, part of a fleet of 50 Navistar diesel vehicles purchased by Montgomery, had problems with electrical panels and parking brakes that caused the fires, officials said.

In an e-mailed statement Wednesday, the National Highway Traffic Safety Administration said it is looking into the buses to “determine if a safety defect exists in these vehicles.” Officials there said they do not know when their investigation will be finished.

The most recent fire happened in March and is still under investigation, Montgomery officials said. No passengers were aboard the buses when the fires occurred. One driver bumped his head as he tried to escape a fire and was treated for minor injuries, according to Montgomery officials.

Montgomery officials said they continue to operate the 45 remaining Navistar buses as “infrequently as possible” for the Ride On service, which provides 26 million passenger trips a year. The county’s bus fleet totals about 300.

Anthony Bruce, 67, of Silver Spring, gets off the #51 Ride-On Bus in 2009 at Glenmont Metro station. (NIKKI KAHN/THE WASHINGTON POST)

The small size of the Navistar buses — about 30 feet long — enables them to navigate neighborhoods and other areas with tight streets. Montgomery officials said they carefully inspect the buses every 5,000 to 6,000 miles for preventive maintenance.

“We will not put a bus on the street that we believe is unsafe,” said David Dise, director of Montgomery’s Department of General Services.

Representatives from Local 1994, a union that represents bus operators, have expressed concern about the safety of their drivers on the buses and have filed a grievance claiming that the county has failed “to enforce safety and health obligations” of the employees and the public and demanding that the problem buses be taken out of service. Montgomery is looking to replace the buses over the next six months, but union officials are still worried.

“That’s not soon enough,” bus operator and union leader Nelvin Ransome said in a statement. “The possibilities are still too great we’ll see more incidents in the next six months.”

Navistar spokeswoman Karen Denning wrote in an e-mail that the bus fires “appear to be isolated to Montgomery County.”

“Navistar has participated in the investigation of the incidents in Montgomery County where our bus chassis were involved and most have resulted in inconclusive findings,” she wrote. “On March 22, 2012, NHTSA opened an investigation on the buses in Montgomery County. No documentation has yet been provided about the questions they intend to ask. Navistar will, of course, cooperate fully with NHTSA.”

Montgomery spent $8.75 million in 2007 to buy the 50 diesel buses.

When the first bus fire occurred in September 2009, Montgomery officials said, they idled the fleet of Navistar buses, made repairs and put the buses back into service in July 2010.

But further incidents occurred, including cases in which the parking brake would activate when the bus was being driven, Dise said.

“There clearly have been a lot of issues with these buses,” Dise said. “It has to do with the wear and tear. These are buses that are not keeping up with the conditions we need them to put up with. They haven’t held up as long as they should have.

“We’re running them as infrequently as we can, but we have a lot of people who rely on bus service.”

Montgomery officials said that the county has spent about $13,700 in repairs to the buses and that the rest of the repair costs have come from Rohrer, the dealer that sold them the buses.

The county is looking to several sources for alternatives. It has spent about $190,000 total buying 15 used buses from the city of Pittsburgh. Officials said the county may also buy used buses from Metro and the city of Philadelphia.

Montgomery eventually plans to buy new, more heavy-duty buses at a cost of $426,180 each, Dise said.

What happens to the 45 Navistar buses once the new buses arrive? One word, Dise said: Scrap.