A prison inmate serving a life term for two murders is running for a U.S. Senate seat in Minnesota. And it's legal.

Minnesota law bars inmates from running for state-level offices, but they can run for federal office. Leonard Richards is keenly aware of the loophole, and he's unsuccessfully sought federal office several times despite serving a prison sentence that offers no parole, the Star Tribune reported.

Richards, 75, was convicted of killing his half-sister, May Wilson, in 1982, and of fatally shooting his lawyer, Robert Stratton, in 1987. Stratton's sister said the Minnesota Secretary of State's Office told her it couldn't keep Richards off the ballot.

Bert Black, a legal adviser for the office, said in an email that despite Richards' "unspeakable acts," courts have ruled that the agency cannot prohibit a felon — even a prisoner — from filing an affidavit of candidacy.

"The only valid requirements are that you be of a certain age and that you live in the state on Election Day, essentially," he said.

Stratton's sister said she wants voters to know Richards' history. She said she knows he won't win the race against U.S. Sen. Amy Klobuchar, but that "even one vote for this murderer is too many."

She also intends to ask lawmakers to change the requirements.

Richards previously ran in the Democratic primary for a congressional seat in 1992 and received more than 14,500 votes. He ran again in 1994 and won more than 4,000 votes. Richards didn't immediately respond to the newspaper for comment.