The department has already had to confront excessive-force problems. After a Washington Post series in 1998 revealed that the city’s officers had shot and killed more people per resident in the 1990s than any other police force in a large American city, the Department of Justice entered into a memorandum of agreement with the city to reform its policing.

“We went through a transformation with regard to use of force when Justice came in here,” Chief Newsham said.

Under this theory, cities that have not had such reviews and calls for accountability may find that cameras have a greater effect.

Criminologists also suggested that the effect of the body cameras diminished over time, and that officers began to behave as they had before they started to wear the devices. Although if that were true the researchers would have found a greater gulf between those with and without cameras at the beginning of the study, which they did not. Or, the effects of the cameras may have spilled over to officers who did not wear them, just because they knew their colleagues did.

“This area of police practice is under-researched, so we really don’t know a lot,” said David A. Harris, a professor at the University of Pittsburgh School of Law who studies police accountability. “In this police department, cameras had a certain effect, but you cannot extend that to other police departments.”

Until now, the most commonly cited study on police body cameras had suggested that they did indeed have a calming effect. That experiment took place in 2012, in Rialto, Calif., where officers were randomly assigned cameras based on their shifts. Over a period of one year, shifts that included cameras yielded half as many use-of-force incidents (including the use of a police baton, Taser or gun) as shifts without cameras did. The number of complaints filed by civilians against officers also declined — by 90 percent compared with the previous year.

The Rialto study had a big impact in policing. Axon (formerly known as Taser International) has sold more than 300,000 police cameras worldwide and cites the Rialto study on its website. A federal judge also cited the study in 2013 when she ordered the New York City Police Department to conduct a yearlong pilot program using body cameras. (The department has outfitted 927 officers with cameras and will compare their performance with officers without; results are due out this spring.)