FARGO, N.D. (AP) _ A judge's role in politically sensitive trials including cases involving Indian activists and the right-wing Posse Comitatus may explain the pipe bomb mailed to him this week, a U.S. attorney said.

Authorities do not know if there is a connection between the bomb and U.S. District Judge Paul Benson's past trials, but the possibility will be explored, U.S. Attorney Rodney Webb said Thursday.

A bomb in a package addressed to Benson was disarmed Wednesday at the Federal Building and appeared to be similar to one that blew up Monday at the post office a block away, slightly injuring four people, authorities said.

''Judge Benson has been involved in several cases even of national note that have involved groups who are single-issue type groups,'' Webb said. ''There can be people within those single-issue groups who become rabid in their views. And that is part of the investigative thrust.''

Benson, 68, returned to work under guard Thursday and turned a bank theft trial he was hearing over to another judge. Webb said the judge was stepping aside from the case for security reasons but that authorities don't believe the defendant had anything to do with the bomb.

Benson declined comment Thursday, according to a person who answered the telephone at his chambers.

The judge, who is semi-retired, was appointed by President Nixon in 1971.

In 1977 he presided over the trial of Indian activist Leonard Peltier, who was convicted of slaying two FBI agents on a South Dakota reservation. The trial was moved to Fargo because of heavy media coverage in South Dakota.

Indian activists accused Benson of being part of a government conspiracy to unjustly convict Peltier.

Three years ago, Benson heard the case of Yorie Kahl, son of tax protester Gordon Kahl, and two other men charged in connection with the slaying of two U.S. marshals in a bloody gunbattle near Medina. The three were convicted.

The elder Kahl, who like his son was a member of the right-wing group Posse Comitatus, was indicted with the other three but not put on trial in Benson's court because he was a fugitive. He later died in a second gunfight with authorities in Arkansas.

After the trial, federal investigators learned of a death threat against the judge.

The judge also has been involved in other controversial cases, including ones dealing with abortion rights, the posting of the Ten Commandments in schools and 11 Indians convicted in the killing of a former police officer who also was an Indian.

Meanwhile, an object that looked like a pipe bomb was found in a postal drop box in Belcourt, about 170 miles from Fargo, on Thursday, but authorities said it was a hoax.