LONDON-- The Libyan rebels' top military commander is dead, and Western confidence in the movement seeking to oust Muammar Qaddafi is shaken.

The killing of Abdel-Fattah Younis, in circumstances that remain murky, is a blow for the U.S., Britain, France and other countries backing Libya's under-trained and divided opposition alliance.

Younis was Qaddafi 's feared security chief and his defection gave the rebels a major boost -- but also left him hated on both sides. On Friday, speculation swirled about whether the regime or his own comrades had killed him, and what the death would mean for the deadlocked civil war.

Britain, one of the major participants in NATO's anti-Qaddafi  bombing campaign, condemned the killing, but was cautious in its response.

"Exactly what happened remains unclear," said Foreign Office Minister Alistair Burt. He said he had spoken to the rebels' political leader, who had stressed that "the killing will be thoroughly investigated."

"We agreed that it is important that those responsible are held to account through proper judicial processes," Burt said.

Younis' death was announced Thursday by the rebels, who gave conflicting accounts of the details -- undermining Western confidence.

Rebel security officers initially said they had arrested Younis for questioning about alleged ties to Qaddafi 's regime. Later, rebel political leader Mustafa Abdul-Jalil said Younis had been summoned for questioning on military matters but was shot dead with two aides before he arrived. He said the rebels had arrested a suspect, but had not found the bodies.

Abdul-Jalil did not say outright who he thought was behind the attack, but appeared to blame the regime, calling on rebel forces to ignore "these efforts by the Qaddafi  regime to break our unity."
Oliver Miles, a former British ambassador to Tripoli, said "the most straightforward explanation" was that Qaddafi  forces had killed Younis -- but that did not make it the most likely explanation.

"He had a lot of enemies," Miles said. "It could be personal, it could be factional within the NTC," the opposition National Transitional Council.

Fawaz Gerges, director of the Middle East Center at the London School of Economics, said that "given the infighting among the rebels, probably some elements that are opposed to him did it."

He said that whoever was responsible, it was "a major blow to the credibility of the rebels."

"Paris or London or Washington are probably extremely anxious about this turn of events," Gerges said. "They are counting on the rebels to put their house in order."

Younis' death is a propaganda coup for Qaddafi 's forces, even if they did not kill him.

Younis was among the army officers who joined the 1969 coup that brought Qaddafi  to power, and remained the dictator's ally for more than 40 years, as a pillar of the country's feared security apparatus.

He was the interior minister and commander of the powerful commando Lightning Brigade when he defected to the opposition early in the uprising that began in February. It was a coup for the rebels and their international allies -- but many mistrusted Younis because of his long ties to the Qaddafi  regime.

His death is a deeply worrying development for the more than 30 countries, including the United States, that have recognized the National Transitional Council as Libya's legitimate government.

Britain this week used even stronger language, calling it the "sole" governmental authority and inviting the rebels to take over the Libyan embassy in London.

The diplomatic moves were an attempt to boost the rebels, who have made little military progress of late, despite the four-month-old NATO bombing campaign directed against Qaddafi 's forces.

The rebels control of much of eastern Libya and pockets of the west, and Qaddafi retains hold over the rest from his stronghold in Tripoli, the capital.

Mahmud Nacua, the rebels' newly named envoy to Britain, on Friday said he couldn't comment on Younis' death, explaining in a brief telephone interview that he didn't have enough information about the case.

The killing opens up the possibility of a tribal split within the rebel alliance. Gerges said reports suggested Abdul-Jalil was urgently "trying to reassure the tribes that the killing of Younis was basically carried out by a rogue unit instead of being sanctioned by the leadership."

France, another key member of the anti-Qaddafi alliance, said it was business as usual for the military mission.

Military spokesman Col. Thierry Burkhard said the French military had not changed its tactics or strategy since the announcement of the death.

"It's a NATO operation, therefore it's NATO's strategy," he told AP. He said the French had not received any new operational orders from NATO since the death was announced.

He suggested that a single individual's absence would not signal a major shift, saying the operation was based on a U.N. mandate "and it does not let individual people feature in the game."
France's Le Monde newspaper took a harsher view, running a front-page editorial headlined

"The Worrying Fragility of the Libyan Opposition." It said the rebels' version of events "is hardly reassuring," and gives reason "to doubt the capacity of the council to exercise power."

"This risks reinforcing Tripoli's hand," Le Monde wrote. "The council, while it hasn't stopped gaining international legitimacy, always gives the image of a disorganized movement. The absence of firm political direction and limited military capacity, despite the support from NATO."

Gerges said much depended on how the rebels handle the murder investigation, and whether they can make a military breakthrough.

"This could be a bump on the road," he said, "or it could be a nail in the coffin of the narrative that the rebels define a different vision for Libya."