BENGHAZI, Libya - The head of the Libyan rebel armed forces was shot and killed Thursday just before arriving for questioning by rebel authorities, their political leader said in a carefully worded statement to reporters that gave few details on who was behind the killing.

Adding to the confusion, the rebels had said hours earlier they had already detained the commander, Abdel-Fattah Younis, on suspicion his family might still have ties to Moammar Gadhafi, raising questions about whether he might have been assassinated by his own side.

Such a scenario would signal a troubling split within the rebel movement at a time when their forces have failed to make battlefield gains despite nearly four months of NATO airstrikes against Gahdafi's forces. It could also shake the confidence of the United States, Britain and several dozen other nations that have recognized the rebel council as Libya's legitimate leaders.

Announcing the killing at a press conference where he did not take questions, Mustafa Abdul-Jalil, head of the rebels' National Transitional Council, called Younis "one of the heroes of the 17th of February revolution," a name marking the date of early protests against Gadhafi's regime.

He said two of the commander's aides, both colonels, were also killed in the attack by gunmen and that rebels had arrested the head of the group behind the attack. He did not say what he thought motivated the killers.

Younis was Gadhafi's interior minister before defecting to the rebels early in the uprising, which began in February. His abandoning of the Libyan leader raised Western hopes that the growing opposition could succeed in forcing out the country's ruler of more than four decades.

Rebel forces, however, held mixed views of the man, with some praising him for defecting and others criticizing his long association with Gadhafi.

Hours before the commander's death was announced, rebel military spokesman Mohammed al-Rijali had said Younis was taken for interrogation from his operations room near the front line to the de facto rebel capital of Benghazi in eastern Libya.

Later, Abdul-Jalil presented a different scenario, saying Younis had been "summoned" for questioning on "a military matter," but that he had not yet been questioned when he was killed.

He also called on all rebel forces to intensify their efforts to find the men's bodies, but did not explain how the deaths were discovered.

Further complicating matters, another security officer, Fadlallah Haroun said before Abdul-Jalil's announcement that security had found three badly burned bodies. Two of them were dead and one was unconscious, Fadlallah said, adding that one was known to be Younis, though they didn't know which one.

"We formed a fact-finding committee to go the scene because we found three bodies that were burned so badly we couldn't tell from the faces who was who," he said.

US and British officials said they were unable to confirm details of the reports but were looking into them.

Abdul-Jalil never clearly said who he thought was behind the attack, but he called on rebel forces to ignore "these efforts by the Gadhafi regime to break our unity."

He also issued a stiff warning about "armed criminal gangs" in rebel-held cities, saying they needed to join the fight against Gadhafi or risk being arrested by security forces.

Since the uprising's start, rebels have gained control of Libya's east and pockets in the west.

In the western Nafusa mountain range southwest of the capital, Tripoli, hundreds of rebels launched a broad offensive against government forces Thursday, seizing three small towns and advancing on others to secure a major supply route near the Tunisian border, rebel spokesmen said.

Four rebel fighters were killed and several wounded while taking the small towns of Jawsh, Ghezaya and Takut, Abdel-Salam Othman said. He said rebels captured 18 government soldiers, as well as weapons and ammunition.