A burning ambulance was on a call Tuesday morning on Benning Road in Southeast Washington. The cause of the fire is under investigation. (Courtesy of D.C. Fire Fighters Association Local 36)

Engine fires in two D.C. fire department ambulances, which are now the subject of a police investigation, were probably caused by malfunctions or by shoddy repair attempts, according to internal incident reports.

Fire department investigators concluded that one of Tuesday’s fires was accidental and that the other, which occurred the same day, was not a full-fledged blaze, just smoke pouring from under the hood. The reports were completed before a deputy mayor last week ordered D.C. police to take over the investigation in order to ensure that “nothing untoward is occurring.”

District officials said the conclusions by fire investigators do not represent the final results of a criminal probe that police expect will take some time. They also said the findings do not preclude the possibility of tampering intended to make the fires appear accidental.

Still, the reports, which were obtained by The Washington Post, provided fodder for the head of firefighters union Local 36, who took exception to the use of the word “untoward.” The president, Edward C. Smith, called the request for a police inquiry an underhanded way of accusing firefighters of sabotaging equipment during a vehement labor dispute.

“I think it was irresponsible for the deputy mayor to make those allegations,” Smith said. “I think the city needs to issue us a formal apology. I’m confident there was nothing done that was untoward.”

Keith St. Clair, a spokesman for Paul A. Quander Jr., the deputy mayor for public safety, said concerns remain about two fires having occurred in ambulances on opposite sides of the city on the same day. Quander, St. Clair said, “wants to know what the facts are” and wants a more thorough investigation than was done initially.

“We’re hoping this investigation shows that these fires were accidental,” St. Clair said.

The union and city have long sparred over equipment issues and slow response times, with each side blaming the other. But the relationship has deteriorated in recent weeks.

On Aug. 8, an ambulance assigned to President Obama’s motorcade ran out of diesel fuel on the White House lawn. City officials said the driver was negligent for failing to fill up the tank, and the union said the city had failed to heed warnings of a faulty fuel gauge.

Last week, District officials acknowledged that maintenance workers had placed aluminum signs in the engines of some ambulances as temporary heat shield because of overheating air-conditioning units. Fire officials ordered them removed after the firefighters union posted a picture on Twitter, and the officials issued a news release calling the repairs “unorthodox.”

Smith called it a prime example of what his members deal with on a daily basis, and he said breakdowns have delayed the response to medical emergencies. City officials counter that many delays, including one in which someone who had a heart attack died and another that stranded an injured police officer, were attributable to excessive absenteeism and dereliction of duty.

So when two ambulances caught fire last week, both sides were quick to raise questions.

St. Clair said that after the second fire, Quander ordered police to conduct an independent investigation. Police said they activated the Arson Task Force, which is made up of local and federal investigators, including some from the fire department. The task force, which generally takes on large and complex blazes, was last used to investigate the multiple-alarm fire at Frager’s Hardware in June.

St. Clair said Quander requested that D.C. fire investigators on the task force recuse themselves from the ambulance investigation. Smith, the union president, said pulling fire investigators off the case “smears their reputation.”

The first of Tuesday’s fires burned the engine of an ambulance on a call in Southeast Washington. The ambulance crew and a D.C. police officer were treating a patient inside a home when the flames shot from the engine. The department’s investigator learned that the ambulance had “ongoing maintenance issues” and that the particular model was “notorious for having air conditioner problems,” according to the report.

The report says the fire started near the air conditioner, and it concludes that the fire was an accident. But the investigator also wrote that he was “unable to identify the specific component failure that led to this fire.”

In a report on the second fire that day, which occurred outside MedStar Washington Hospital Center in Northwest, the investigator wrote that he found a plastic container of transmission fluid stored in the engine compartment.

The driver of the vehicle said the air conditioner had not worked properly all day and that it suddenly blew hot air before smoke appeared from under the hood, according to the report. Because the incident was relatively minor, the investigator did not give an official opinion on whether it was accidental.

The fire department also looked into an Aug. 2 ambulance fire outside Washington Hospital Center. That fire seemed to originate in the battery, and it caused $5,000 in damage to the $120,000 vehicle, according to an internal report.

The fire investigator determined that an electrical problem in the battery compartment caused plastic to ignite, the report says. The report notes that the driver had experienced problems with the emergency lights and that the battery was not the one that came with the vehicle.