Reuters) - U.S. authorities are investigating possible collusion between militants who launched a deadly attack on the U.S. consulate in Benghazi, Libya and locally hired Libyan personnel guarding the facility, three U.S. officials said. So far there is no proof that the attackers, who killed U.S. Ambassador Christopher Stevens and three other U.S. officials, were helped by Libyan security personnel hired by the consulate. One official said the Obama administration was playing down this possibility.However, all the officials said that the question of whether the attackers had inside help or advice was a serious issue in the U.S. investigation into...