The investigation into last week's killing of four American diplomats in eastern Libya is raising pressure on a shaky new Libyan government to quash pockets of violent Islamists emerging after the country's revolution and now threatening its struggle for stability.

Late Monday, one of the country's top security officials met with two leaders of the militant Islamist group Ansar al Sharia. Ansar has become a primary target in the search for those responsible for last Tuesday's deadly attack on the U.S. Consulate here—especially after U.S. and Libyan officials said some of the group's members were in communication with Al Qaeda ahead of the battle.

Libyan military Chief of Staff Yousef al-Mangush met with two Benghazi-based leaders of the group at a Defense Ministry office in this eastern city, according to group representatives and an aide to the chief of staff. It wasn't immediately clear what was discussed during the meeting, which broke up around midnight after about three hours.

Libyan officials investigating the deaths of U.S. Ambassador Christopher Stevens and three embassy colleagues haven't disclosed whether any of the 50 people arrested so far are from Ansar al Sharia. The group itself has repeatedly denied it was responsible for organizing or directing the attack on the consulate.

Ansar spokesman Hani Mansouri told a Benghazi radio station on Sunday that Ansar was forming its own committee to investigate the attack, saying it would sue organizations, including "foreign media," that had defamed the organization by associating it with the incident.

The spotlight on Ansar highlights what has been a slow but significant splintering over the past year of Libya's shrinking Islamist ranks—a process in which most onetime jihadist and ultraconservative groups renounced violence and threw in their lot with the government, while a few, such as Ansar, appeared to have moved further into the fringe and turned more violent.

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