Japan is negotiating with the United States the possibility of transferring a U.S. Navy officer, convicted of a fatal car crash in 2021, to his home country at the request of his family and U.S. lawmakers, diplomatic sources said Sunday.

The government of Prime Minister Fumio Kishida has claimed there were no flaws in the criminal proceedings against U.S. Navy Lt. Ridge Alkonis, 34, but criticism has been growing over his long detention in Japan, with some members of the U.S. Congress also showing their support.

Foreign affairs experts said Japan-U.S. relations would worsen if his detention is prolonged, as President Joe Biden, who was lobbied by Alkonis' family, has expressed willingness to settle the case as soon as possible.