Fairfax County Police Chief Edwin C. Roessler Jr. said Tuesday he suspended an officer who cooperated with federal deportation officers by turning over an illegal immigrant he encountered during a traffic stop — a move the chief said violated department policy.

Chief Roessler didn’t identify the officer but said he “deprived a person of their freedom” by cooperating with U.S. Immigration and Customs Enforcement.

The chief called that unacceptable.

“Our county is one of the most diverse counties in the nation and no one should have the perception that FCPD is acting as a civil immigration agent for ICE,” he said in a statement. “This matter damages our reputation and the longstanding policy that I have stated many times that our officers shall not act as immigration agents.”

The chief said the officer was working a traffic accident in the Huntington section of the Virginia county on Sept. 21 and came across someone without a driver’s license. When the officer ran the name, it returned a flag from ICE, which said the immigrant was a fugitive after failing to appear for a deportation hearing.

The county’s system confirmed the warrant. The officer called, and an ICE employee responded.

Chief Roessler said the county officer should not have detained the immigrant to turn over to ICE. He said that move broke department policy, which tells officers not to bother to confirm administrative warrants through the county’s system. The majority of administrative warrants are from ICE.

“The officer involved in this event has been relieved of all law enforcement duties pending the outcome of this investigation,” the chief said.

Luis Aguilar, director of CASA Virginia, a leading advocacy group for immigrants, said Chief Roessler’s decision to suspend an officer was bold — and correct.

“We think it’s a very appropriate action,” Mr. Aguilar told The Washington Times. “This is local law enforcement, this is the local police department, and they cannot be enforcing federal immigration laws. … This is a clear message of where and how the chief of police thinks.”

ICE did not respond to a request for comment, but the agency’s acting director last week used a press conference at the White House to complain about communities that refused to cooperate with his deportation officers.

Matthew T. Albence, the acting director of ICE, said police departments routinely cooperate with other departments on warrants and detention requests and there is no reason they should treat ICE — a federal law enforcement agency — differently.

“I guarantee you, I can go into Fairfax County [court] today and there will probably be sheriffs from Loudoun County, sheriffs from Prince William County, maybe a couple of marshals guys that are there waiting for somebody,” he said. “It’s a common occurrence in law enforcement. The only reason it’s being made controversial is because politicians are looking to exploit it.”

Jessica Vaughan, policy studies director at the Center for Immigration Studies, said Fairfax County’s police department is getting the law enforcement equation backward.

“The police chief is ordering an investigation of an officer who did exactly the right thing by responding to an active ICE warrant on an illegal alien who was a fugitive,” she said. “It’s astonishing that a police chief is more intent on punishing his officer than seeing to it that legitimate laws are enforced.”

She disputed the police department’s suggestion that the county officer was acting as an agent of ICE. She said that is no more true than if he had held someone for a warrant issued by neighboring Montgomery County in Maryland.

She said Fairfax police intended to shame the officer.

Chief Roessler said the illegal immigrant in question was picked up by ICE and has been processed and released on an ankle bracelet pending the outcome of deportation proceedings.

Mr. Aguilar said it’s likely the migrant will be deported and the police officer will be responsible.

“We don’t even know about the family itself, who’s going to be separated. We don’t know if there’s kids involved,” he said.