DETROIT — Members of a Christian militia accused of plotting an antigovernment uprising were acquitted on sedition and conspiracy charges on Tuesday by a federal judge who said prosecutors had failed to prove that the group had concrete plans to attack anyone.

Judge Victoria A. Roberts of Federal District Court dismissed all charges against five members of the Hutaree militia, who the authorities claimed had plotted to kill a police officer and then ambush those who attended the funeral. She dismissed the most serious charges against the group’s founder, David Brian Stone Sr., and his son Joshua, but said they must remain on trial for some lesser weapons charges. All of the defendants had faced possible life sentences if convicted.

“The government’s case is built largely of circumstantial evidence,” Judge Roberts wrote in her ruling. “While this evidence could certainly lead a rational fact-finder to conclude that ‘something fishy’ was going on, it does not prove beyond a reasonable doubt that defendants reached a concrete agreement to forcibly oppose the United States government.”

The acquittals come two years after the police in southern Michigan, Ohio and Indiana rounded up the militia members, some of whom have remained in custody since then. The Federal Bureau of Investigation had used an undercover agent and paid informer to infiltrate the Hutaree for more than a year.