 Through the efforts of the British Minister in Washington, his sentence was commuted to life imprisonment at the isolated Fort Jefferson military prison located about 70 miles west of Key West, Florida, in the Dry Tortugas islands of the Gulf of Mexico.He arrived at Fort Jefferson on October 8, 1865.The great majority of the 527 prisoners at Fort Jefferson when Grenfell arrived were Union Army privates whose most common transgression was desertion.