The only prisoner ever convicted in the United States of having ties to the Sept. 11, 2001, attacks has applied to serve out his life sentence in his native France, stirring a protest by Republican senators.

The prisoner, Zacarias Moussaoui, 56, was arrested in Minnesota a month before the hijackings, which killed nearly 3,000 people in New York, at the Pentagon and in Pennsylvania. For a time after the Sept. 11 attacks, U.S. officials theorized he was the would-be 20th hijacker in the attacks carried out by 19 men, but later dropped that assertion.

In 2005, he pleaded guilty to conspiring to kill American citizens in a federal court case in Alexandria, Va. A jury sentenced him the next year to life imprisonment, rather than the death sentence prosecutors had sought. He is held in solitary confinement at the federal supermax prison in Colorado, with no possibility of release.

Nicole Navas Oxman, a Justice Department spokeswoman, declined to confirm the fact of Mr. Moussaoui’s transfer request, citing department policy, but suggested it would not be granted.

“Zacarias Moussaoui is serving a life sentence following conviction for terrorism offenses,” she said. “The Department of Justice plans to enforce this life sentence in U.S. custody.”

But in a July 10 notice to relatives of people who were killed in the attacks, the department invited comments on the request within 30 days.