Thousands of Libyans turned out in rebel-held Benghazi on Friday for the funeral of the opposition's military chief.

Abdel Fattah Younes and two of his aides were gunned down on Thursday under mysterious circumstances. The military chief had been part of Libyan leader Moammar Gadhafi's inner circle before joining the rebels earlier this year.

The opposition The National Transitional Council says it is investigating the killing. It has said only that Younes was gunned down on route to Benghazi, where he had been summoned to discuss “a military matter.”

Earlier, rebels said they had detained Younes on suspicion his family may still have ties to Mr. Gadhafi's government.

U.S. State Department spokesman Mark Toner said Friday that Younes' death will pose a challenge to the rebels because of his military expertise and leadership skills.

Younes had been the Libyan leader's interior minister and one of his closest confidants before unexpectedly defecting to the opposition early in the uprising, which began in February.