FBI has been unable to get into Libya to investigate consulate crime scene But the inability to get to Benghazi immediately is only the first of many enormous challenges that American authorities face in fulfilling President Obama's vow to seek justice for the attack. Even after the FBI is able to set up in Benghazi and begin trying to reconstruct what happened during a four-hour melee, the agents will face problems more complicated than they encounter at domestic crime scenes, according to law enforcement sources who spoke on condition of anonymity because they were not authorized to talk to the press. They will have to secure a charred crime scene that was left open to the public and looted by militants. The agents will have to sift through the ashes for shell casings and other physical evidence. In addition, they will have to track down people who were part of the initial protest, which could mean venturing into unfamiliar, dangerous territory in a country with little security, according to according to several former and current FBI agents who have been involved in similar investigations in foreign countries. One of the problems will be that the central government in the Libyan capital of Tripoli does not exert authority over the many armed militias and some of these groups are known to be sympathetic to al-Qaeda. "The obstacles are huge," said Donald Borelli, an FBI agent for 25 years and the former assistant special agent in charge of New York's Joint Terrorism Task Force. "It's not like you can walk in like in the movies and say, "FBI, we're here," Borelli said. The FBI doesn't mean anything in their country. It's not like your badge carries any weight. Libyan authorities said they have made four arrests in connection with the attack, but they have provided no other details. Federal agents who have been involved in past investigations overseas cautioned that it was too early to know the significance of the arrests, and it was unclear if the people arrested were actually involved in the violence. "These could be the right guys," Borelli said. But it is way too early to do a victory dance. My experience is that sometimes local authorities in these countries will arrest the usual suspects, but in the end they may have nothing to do with the case. Borelli led the team of agents who investigated the 2010 bombings in Kampala, Uganda that killed 76 people during the final game of the World Cup. Local authorities initially arrested scores of people, he said. But in the end only a few men were actually found to be responsible for the attacks. Ali Soufan, a former FBI special agent who led the investigation into the bombing of the U.S.S. Cole in 2000 and was involved in several other high-profile terrorism cases, said that a key hurdle for the team going into Libya is security. "You want to solve the case, but at the same time you have to protect your people," said Soufan, the author of "The Black Banners: The Inside Story of 9/11 and the War Against al Qaeda." Julie Tate contributed to this report