In the last week of his life, Ambassador J. Christopher Stevens was focused on a problem that has bedeviled Libya since the overthrow of Moammar Gaddafi last year: the countless militias that operate above the law.

Despite the formation of a new government and successful elections this summer, many of Libya’s streets are still run by former rebel fighters who have not fallen under the command of a central authority.

It remains unclear who was behind the attack on the U.S. Consulate in Benghazi on Tuesday that killed Stevens and three other Americans. But the extent of the lawlessness that pervades Libya was certainly a factor.

Stevens, who became ambassador in May, had been talking with friends and colleagues about solutions. According to a friend who attended meetings in Benghazi with Stevens on Monday and Tuesday, the diplomat had proposed sending militia leaders to the United States in an exchange program that would allow them to meet American Muslims and learn about U.S. democracy. The friend, who dropped Stevens off at the diplomatic compound just hours before he died, spoke on the condition of anonymity out of concerns about personal safety.

Libyan authorities said Thursday that they had made four arrests in connection with the attack, but they provided no other details.

In acknowledging Libya’s security shortcomings, the country’s deputy U.N. ambassador, Ibrahim Dabbashi, said Wednesday: “We have to say the reality — that the authority of the government is still not covering the whole territory of Libya, and there are some groups and persons who are outlaws, and the government could not at this moment contain all of them.”

More than 200 private militias are still active in the sprawling North African country, despite efforts by the country’s nascent democratic leadership to draw them into more centralized units, according to a study released this week by the Atlantic Council.

Many of the militias control swaths of territory and huge weapons arsenals looted from Gaddafi’s military bases, according to the study. The stockpile includes tanks, antiaircraft guns and rocket launchers of the type that hit the U.S. Consulate, and about a dozen of the militias are able to project significant military power.

Particularly worrying to U.S. intelligence officials is the possibility that the groups have acquired the far more lethal man-operated portable air defense systems, or MANPADs, which are capable of taking down an airplane. A senior Obama administration official, speaking on the condition of anonymity because the project is sensitive, said U.S. intelligence estimates that 100 to 1,000 MANPADs are still unaccounted for, despite U.S. efforts to buy up and destroy the weapons. Intelligence officials have speculated that the missiles were also smuggled across Libya’s borders.

Glen Doherty, a former Navy SEAL and one of the Americans killed in Tuesday’s attack, told ABC News last month that he had gone into Libya to track down MANPADs as a contractor for the State Department.