BEIRUT, Dec 20 (Reuters) - Germany has secretly released a Hizbollah member jailed for life for killing a U.S. Navy diver and returned him to Lebanon despite an extradition request from the United States, Lebanese political sources said on Tuesday. They said Mohammad Ali Hammadi, convicted of killing Navy diver Robert Dean Stethem during the 1985 hijacking of a TWA flight to Beirut and sentenced to life without parole, was flown back to Beirut last week.