The conflict in Libya is about to get a whole lot messier. Rumors swirled all day Thursday that the chief of staff of the rebel military forces, Abdul Fatah Younes, had been arrested. Then, the bombshell came in a late-night press conference in a Benghazi hotel: Younes had been killed, along with two other rebel officers. The announcement came from Mustafa Abdul Jalil, head of the rebel National Transitional Council, who claimed that the leader of the group that carried out the shooting was in custody but the bodies of Younes and the two officers had not been found. The reaction was immediate. Armed gunmen supporting Younes hit the streets of Benghazi and blasted out windows at the hotel where the conference took place.

Younes's death leaves a dangerous power vacuum in the military leadership of the rebels and could lead to in-fighting among various factions. Younes had been jockeying for power with a rival military commander named Khalifa Heftar who returned to Libya from the U.S. in March. Even more troubling, Younes’s death could inflame tribal tensions. Younes was a member of the Obeidi tribe, one of the largest and most powerful in eastern Libya, and it will take a lot of delicate negotiations to convince his clansmen to stand down.

Younes has long been a controversial figure in the rebel leadership. He was one of the officers who joined Muammar Gaddafi in a coup against the monarchy in 1969, and served alongside him for over 40 years. His most recent post was as Gaddafi's interior minister, a job that gave him oversight of the detention and torture of thousands of political dissidents. He switched sides and joined the rebels in February.

In recent months, some rebel grunts questioned Younes's loyalty and asked why he didn't join them on the frontlines in eastern Libya more often. Other rebel leaders seemed skeptical, too. In an interview with a handful of journalists in March, Ali Tarhouni, the finance minister in the National Transitional Council, was asked whether Younes was the right person to command the rebels. After a long pause, he answered, “I'm not sure we have somebody better.” For his part, Younes addressed questions about his loyalty in several press conferences and claimed that rebel forces would have never succeeded in the east if he hadn’t switched sides.

In the conference on Thursday night, Jalil called Younes “one of the heroes of the February 17 Revolution.” But he also confirmed to reporters that Younes had been called back to Benghazi from frontline positions near the oil city of Brega for questioning. The more important point that Jalil didn't address is how Younes could have been ambushed when he traveled with a very large security detail. That raises questions whether Younes was shot by someone within the rebel ranks. It's also puzzling how Jalil could have confirmed Younes’s death if his body had not been found.

The assassination poses a fresh challenge to the rebels at a time when they finally seemed to be gaining some political and military momentum. The British government announced on Wednesday that they would give diplomatic recognition to the rebel Council and give them access to $149 million dollars in frozen Libyan oil funds. And rebel military commanders in western Libya claimed they had taken control of three towns from pro-Gaddafi forces in a large offensive on Thursday.

All of the hard-fought gains could quickly crumble if the rebels begin to turn their guns on each other.