What began as a routine arrest during President Trump’s crackdown on crime ended with nearly a week in jail.

On Aug. 20, after noticing federal agents on patrol, Treyvon Cooper, 28, exited his car parked at a gas station in Northeast Washington and headed into a nearby store, according to court papers. The agents, suspicious, searched his vehicle and found a marijuana joint still fuming, along with more cannabis in his glove box.

He was charged with a misdemeanor, unlawful possession of marijuana, but he remained behind bars for four days more than the 48 hours permitted by law before a court hearing.

Mr. Cooper’s experience underscores a repeated pattern since National Guard troops and federal agents fanned out across the streets of Washington: Some defendants have been imprisoned for far longer than they should have been, in what experts say is a violation of the law. According to court records reviewed by The New York Times, at least 11 people who were charged with federal crimes by the U.S. attorney’s office in Washington have stayed in jail beyond the 48-hour maximum without an initial appearance before a judge to determine whether there exists enough evidence for criminal charges.

The pattern has raised widespread concerns among defense lawyers, former prosecutors and judges, who warn that the U.S. attorney’s office in Washington, led by Jeanine Pirro, is sidestepping crucial safeguards in service of Mr. Trump’s tough-on-crime messaging. Some suspects, public defenders say, have spent days in extended imprisonment, detained in jails cells that are frequently in poor condition.

“It’s hard for me to conceive of a U.S. attorney who’s so unethical that they would say, ‘Let’s slow-walk these people’ to send a message that there’s a new sheriff in town,” said Glenn Kirschner, a former prosecutor who served at the U.S. attorney’s office in D.C. for more than 20 years.