LONDON — British police arrested a man Tuesday on suspicion of manslaughter as they searched for answers about why a cargo ship hit a tanker transporting jet fuel for the U.S. military off eastern England, setting both vessels ablaze. One sailor was presumed dead in the collision, which sparked fears of significant environmental damage. Humberside Police said the 59-year-old was detained “on suspicion of gross negligence manslaughter in connection with the collision.” The man, who was not named