The FBI’s policy, which is unusual among law enforcement agencies, was adopted after the Sept. 11, 2001, terrorist attacks. Known as “up or out,’’ it requires FBI supervisors to leave their posts after seven years and compete for other managerial jobs, retire or accept a demotion in the same field office with lower pay.

FBI officials say the term limits have brought strong managers into hundreds of positions created in the years after Sept. 11. But the plan to retain Mueller has revived long-simmering tensions over the policy, which some say has robbed the bureau of veteran supervisors who retired because they did not get promoted…

“People are up in arms about this,’’ said one agent, who likened the news to “a shot in the kneecaps.’’

“We have lost valuable experience,’’ the agent said.