D.C. police officers wearing body cameras reported using force about as often as colleagues who didn't have them, and citizen complaints against the two groups were about even, according to a new study that bucks early expectations about the impact of the devices.

When the cameras started to appear in police departments in 2014, experts predicted behavior on both sides of the badge would improve under the watchful eye of the lens. But the look by the District's in-house research branch suggests otherwise — a finding that could shift the debate on one argument used to put the cameras in virtually every big-city police department nationwide.

D.C. Police Chief Peter Newsham said the results surprised department leaders and were "not what we anticipated." He said it appears in many police interactions, "cameras didn't make a difference."

The chief said the recordings have been a valuable tool, providing a precise record of tense and difficult encounters, including police shootings.

The District says the study of its $5.1 million program is among the more comprehensive looks at whether police-worn cameras affect behavior by officers and the people they encounter.

Police body cameras became seen as a key tool for reform after growing concern over the deaths in several cities of people in police custody. Public officials quickly heeded calls to infuse new levels of public accountability and transparency into everyday police work.

[Issues over Ferguson lead push for police body cameras]

Though some police departments were reluctant, most contended the videos would most often exonerate officers facing allegations of misconduct, provide the public with a unique perspective of officers' work and be an invaluable tool for training. D.C. police, and the labor union, embraced the program.

The D.C. research looked at a period when the police force was rolling out its body camera program — and some officers had the cameras while others were still waiting.

Researchers found slightly more officers with cameras reported using force than those without. More people filed complaints against officers wearing cameras than without. The research team said the differences were statistically insignificant, making the influence of the cameras a wash.

Police agencies "should not expect dramatic reductions in the use-of-force complaints, or other large-scale shifts in police behavior solely from the deployment of this technology," concluded David Yokum, who directs The Lab @DC, which conducted the study with assistance from outside universities.

In an interview, Yokum added, "So if you are a police department thinking that this technology on its own is going to be something to cause big shifts on those two dynamics, this would be a cause to recalibrate your expectations."

Newsham, the chief, said the biggest benefit of cameras has been having a clear record in controversial incidents, such as a Dec. 25, 2016, fatal police shooting of a man during a domestic dispute. Relatives argued police shot an unarmed man; the body camera video showed the man with "a rather large butcher's knife," Newsham said. "In today's environment in policing, having legitimacy is something we have to have."

Sgt. Matthew Mahl, chairman of the D.C. police labor union, said he too expected a bigger impact from the cameras.

"I honestly thought that complaints would have come down," he said. "We're spending all this money to realize that everything is the same. Maybe that's a good thing, that we've been doing things right from the beginning."

The union leader said officers appear to have adjusted to the cameras. "It really has been business as usual," he said.

Many departments, including the District, publicly release footage of some police-involved shootings and other encounters, though policy differs from agency to agency. Authorities in Las Vegas released video of officers streaming into the crowd as a gunman fired into a concert earlier this month, killing 58 people and injuring hundreds.

Officers also have been caught in compromising and embarrassing positions. In Baltimore, officers targeted in a corruption investigation implicated themselves on camera.

There have been incidents where critical interactions were not recorded because officers failed to turn on their cameras.

In one incident in the District, an officer did not turn on his camera before he fatally shot an unarmed motorcyclist in a case that has angered the victim's family and friends. The D.C. police union says it will push for a new device that turns on body cameras when an officer removes a gun from the holster.

The 2½ -year study of 2,224 D.C. officers — 1,035 without cameras, 1,189 with cameras — started in June 2015, when cameras were distributed in limited numbers in an initial rollout, and it continued as more officers were phased in to the program. The department reached full deployment of 2,600 cameras in December 2016.

[Debate rages over public access to police body cameras]

Researchers found 880 officers with cameras reported using force during an encounter over a seven-month period. The number dropped to 807 for officers without cameras. Citizens filed 337 complaints against officers wearing cameras and 280 against officers not wearing cameras.

Yokum said the outcomes of internal investigations of those complaints were roughly equivalent to past years.

Michael G. Tobin, director of the D.C. Office of Police Complaints, a civilian review board that supplied researchers with some of their data, said while the cameras have made it far easier to reach conclusive results, the videos have "not had a direct effect yet on our findings."

He said preliminary data shows roughly the same number of complaints were dismissed in fiscal 2017 for a variety of reasons involving officers wearing and not wearing body cameras.

Unlike the report's researchers, Tobin's investigators — who examine allegations of improper use of force and other issues — have watched many hours of body camera videos. Tobin said in volatile, emotional situations, "it's reasonable that people react the same whether they are on camera or not." But, he said, "in routine encounters, when people know the camera is active, I believe we see people acting differently — more professionally, more formally."

Michael D. White, a professor in the School of Criminology and Criminal Justice at Arizona State University, said early expectations about body-worn cameras relied largely on the study of a single, small West Coast police department that at the time was among a handful of agencies with the program. The study correlated a drop in complaints and uses of force to the cameras.

That department, White said, had many problems, and the camera program was among the reforms that could account for the declines. He said additional data as more departments add cameras is more in line with what the District found.

Yokum, the D.C. researcher, said the study does not suggest the camera program is a bust, but he said chiefs should not expect wholesale drops in complaints or in officers using force.

"There are other potential benefits from the program," the researcher said, "such as how the video footage is used in the courts, how it's used to train officers, and the public perception of trust."