A six-week police investigation into fires aboard three D.C. ambulances in August — two on the same day — found no evidence that they were intentionally set, according to the results of an investigation that is to be made public Wednesday.

The findings by police largely support the conclusions reached by fire investigators within days of the incidents — that the fires were most likely accidents caused by a variety of engine problems, such as fuel leaks or electrical malfunctions. The precise causes were not determined.

Deputy Mayor for Public Safety Paul A. Quander Jr. ordered the police review to ensure that “nothing untoward” had occurred in what he described as an unusual number of fires in a a short time.

Although Quander said he was not assigning blame, the head of the firefighters’ union took issue with the comments, saying the deputy mayor had all but accused firefighters of sabotaging their vehicles during a bitter labor dispute. The union had said the fires were another example of shoddy and poorly maintained equipment.

Mayor Vincent C. Gray’s administration should apologize for “accusing the firefighters of untoward acts,” said Edward Smith, the union’s president.

A burning ambulance is seen this past summer in Southeast Washington. (Courtesy of D.C. firefighters Local union #36)

The report was obtained by The Washington Post; it is scheduled to be released publicly Wednesday. Fire and police officials declined to comment ahead of its official release. In August, The Post obtained internal fire investigative reports that reached the same conclusion that police have.

The fires Aug. 2 and Aug. 13 came during particularly heated exchanges between the union and fire officials over whether public safety was being compromised because of too few paramedics and other issues. The police investigation, which spanned weeks, involved experts dealing with arson and vehicle maintenance and interviews with firefighters.

The first fire, Aug. 2, occurred outside MedStar Washington Hospital Center in Northwest Washington. Police said it appears that small fire was caused by an electrical malfunction.

That incident didn’t generate any publicity, but a larger fire Aug. 13 did. That fire occurred outside a Southeast Washington home while a woman inside was being treated. Flames leapt from the ambulance’s engine and caused significant damage to the $120,000 vehicle.

The D.C. firefighters’ union posted a photo of the burning ambulance on Twitter, further damaging the image of a troubled department and sparking the fire chief, Kenneth B. Ellerbe, to respond to the scene. A firefighter accused Ellerbe of assault, alleging that the chief ripped a cellphone from his hand in an attempt to uncover the photographer. Prosecutors declined to take the case.

Police reached the same conclusion that fire investigators did — the fire was cause by a fuel leak or component break that allowed fluid to spread and catch fire. A third small fire that same day, again outside Washington Hospital Center, was caused by an electrical short in an ambulance that had a long history of problems with its air conditioner, police found.