A Charles County ambulance that caught fire and was destroyed on Monday outside Southern Maryland Hospital Center was the seventh rescue vehicle built on a similar Ford chassis to burn in the last month, federal officials said yesterday.

The National Highway Traffic Safety Administration and the Prince George's County Fire Department are investigating the cause of the fire at the Clinton hospital.

The Ford Motor Co. and ambulance companies that customize Ford Econoline E-350 chassis for use as ambulances in many areas nationwide announced a plan last month to correct fuel tank problems that have been blamed for a number of injuries and fires. At least two cities have limited their use of the vehicles as a result of fires.

Work was done last week to modify the $60,000 ambulance that burned about 3 p.m. Monday, said Jeff Brown, president of the Charles County Association of Emergency Medical Services.

The safety administration sent an investigator yesterday to examine the ambulance, used by a volunteer fire company in Waldorf, Md. Ron DeFore, a spokesman for the agency, said it would be premature to conclude that the vehicle burned "as a result of the recall work" or because the modifications do not correct problems in the vehicles.

As of yesterday, DeFore said, the agency had received 210 complaints about problems with gas tanks or fuel lines from operators of ambulances equipped with the Ford chassis. The owners reported 11 fires and 14 injuries related to the fuel problems, he said.

Consumer advocate groups have said problems have been caused by fuel tanks overheating, fuel leaking and gasoline gushing out under pressure when the gasoline filler cap is unscrewed. Fires have occurred after gas that built up in the fuel system was expelled from the gas line, they said.

A Ford spokesman said that about 22,000 ambulances equipped with the E-350 chassis are in service across the country. Most Washington area jurisdictions and private ambulance companies use E-350 ambulances, officials said.

At least one of those, belonging to the Hillandale Volunteer Fire Company in Montgomery County, caught fire in September. A Falls Church ambulance spilled fuel outside the emergency room at Fairfax Hospital last summer.

Prince George's and Anne Arundel counties do not have any E-350 ambulances, spokesmen said. Prince William and Fairfax counties use several of the vehicles, but have not had problems with them, officials said.

On May 22 in Lexington, Ky., an ambulance parked outside a nursing home burst into flames and the fire spread to the nursing home. There were no injuries, a local fire official said.

The Cincinnati Fire Department pulled six of the ambulances -- almost half the city's fleet -- out of service after one of the vehicles caught fire on May 16. Louisville's Emergency Medical Services parked half of its fleet after a fire destroyed a Jefferson County, Ky., ambulance May 26.

"We've had problems with the fuel systems for years," said Brad Learns, a spokesman for Louisville Emergency Medical Services, who said his department was switching from gasoline to diesel-powered ambulances to avoid the problem.Staff writer Laura Bischoff contributed to this report.