Federal prosecutors said on Thursday that misdemeanor charges against people who had protested outside an immigration detention facility near Chicago would be dismissed. It was a stunning end to one of the Trump administration’s most highly publicized prosecutions of demonstrators during an immigration crackdown last year.

In a hastily called hearing in federal court in downtown Chicago, Andrew S. Boutros, the U.S. attorney for the Northern District of Illinois, said that conduct by prosecutors during the grand jury process led to the dismissal. It was uncertain exactly what had occurred, but the announcement followed a closed-door hearing between the judge and lawyers.

“No one acted with the intent to mislead your honor,” Mr. Boutros said.

The trial had been expected to begin with jury selection Tuesday. The case was dismissed with prejudice, federal officials said, meaning that new charges cannot be sought.

In October, the grand jury indicted six people, including a Democratic congressional candidate, on charges of interfering with a federal agent during a protest outside an immigration detention site.

The protest took place on Sept. 26 in Broadview, Ill., a suburb of Chicago, amid an immigration crackdown that Trump officials had called Operation Midway Blitz. Thousands of masked, camouflaged federal agents blanketed the Chicago area in the fall, sweeping up thousands of people accused of violating immigration law and randomly questioning U.S. citizens of their legal status.

This is a developing story. Check back for updates.