For more than three decades, Sheriff Chris West of Canadian County, Okla., a large man whose uniform often includes a Stetson hat, a vest and a gold star badge, devoted his life to law enforcement.

A U.S. Marine Corps veteran, he spent 28 years as a highway patrolman in Oklahoma, working his way up to captain before being elected sheriff of his native county in 2017. He earned the accolade “Oklahoma Sheriff of the Year” in 2019, and won a second term last fall, after running unopposed.

Then came Jan. 6.

Mr. West said he set his badge and his official role aside when he drove to Washington to support President Donald J. Trump. “I went as a citizen, as Chris West, the individual,” he told a news conference in El Reno, the county seat, after he returned.

By his own account, he marched on the Capitol waving a Trump flag and hollering slogans like “Stop the Steal!” and “We love Trump!” But he said that he did not participate in the storming of the Capitol, and he condemned the attack.

His actions have divided Canadian County, which includes parts of Oklahoma City and the rural areas to its west, with several thousand people signing a petition demanding his removal and even more endorsing a counterclaim supporting him.

He is one of at least 30 police or other law enforcement officers who attended the demonstration on Jan. 6. Many are now facing internal investigations and three have thus far been arrested on federal charges related to breaching the Capitol.

Their presence has brought to a boil questions that have been simmering for years: How many law enforcement officers nationwide subscribe to extreme or anti-government beliefs, and how, precisely, can agencies weed them out? Leaders in law enforcement say that public servants must be held to a higher standard than private individuals when it comes to accepting the results of an election and performing their duties.

Police chiefs from the largest North American cities, meeting in an online conference this past week, agreed to work together to try and block members of far-right organizations or others with radical views from entering their ranks.

“There is zero room, not only in society, but more so in professions of public trust and service, for people to have extremist views, regardless of ideology,” said Art Acevedo, the Houston police chief and president of the Major Cities Chiefs Association, which includes senior police officials from almost 90 American and Canadian cities. President Biden’s goal of addressing domestic extremism will partly hinge on the ability to curb its spread in police departments and the military, experts noted.

Concerns about extremism in police ranks have long existed, but after Sept. 11 chasing jihadists took priority over chasing domestic threats, senior police officials and law enforcement experts said.

In recent years, police or other agencies in Virginia, Florida, Nebraska, Louisiana, Michigan and Texas have all fired officers belonging to the Ku Klux Klan. In Philadelphia in 2019, the Police Department announced that 13 officers would be dismissed among the 72 who were placed on administrative leave because of racist Facebook posts.