A team of FBI agents assigned to investigate the deaths of four Americans in Libya has not been able to get into the country because of the volatile situation there, according to law enforcement officials.

The team’s inability to get to the scene, secure vital forensic evidence and interview witnesses means that the task of identifying and prosecuting the assailants will be harder, according to former and current FBI agents.

The delay is probably only the first of many enormous challenges that U.S. authorities face in fulfilling President Obama’s vow to seek justice for the deaths of U.S. Ambassador J. Christopher Stevens and three other Americans in the attack on the U.S. Consulate in the Libyan city of Benghazi.

Agents will have to help secure a charred crime scene that was left open to the public and looted by militants; sift through the ashes for shell casings, residue from explosives and other physical evidence; and track down people who may have witnessed or were part of the attack, which could mean venturing into unfamiliar, dangerous territory in a country with little security, according to several former and current FBI agents who have been involved in similar investigations in foreign countries.

The central government in the Libyan capital of Tripoli does not exert authority over the many armed militias, some of which are known to be sympathetic to al-Qaeda, further complicating the task.

“The obstacles are huge,” said Don Borelli, an FBI agent for 25 years and the former assistant special agent in charge of the FBI’s Joint Terrorism Task Force in New York.

“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.”

The bureau declined to comment. But U.S. intelligence officials said that, despite the delay in getting into Benghazi, agencies have been able to draw intelligence from an array of sources, including news footage of the attack, intercepted phone calls and e-mails, and information from human sources recruited by the CIA.

The officials said they have reached a tentative conclusion that the assault was carried out by a group aligned with al-Qaeda but not directed by the terrorist network’s core leadership. The officials stressed the preliminary nature of the assessments, noting that a massive analytic effort involving every agency in the intelligence community is still in its early stages.

“We still don’t assess that this is core al-Qaeda,” said a senior U.S. intelligence official, speaking on the condition of anonymity because of the sensitivity of the matter. “We assess that this is most likely a group best described as al-Qaeda sympathizers.”

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 unclear whether the people arrested were involved in the violence.

The FBI has the authority to investigate the deaths of Americans who are attacked or killed abroad, and it has done so in many cases around the world, including the 1998 embassy bombings in Kenya and Tanzania, the USS Cole bombing in 2000 and the 2008 terrorist attacks in Mumbai, India.

The FBI’s New York field office is tasked with sending agents to investigate American deaths in Africa. The team, which could eventually include as many as 40 agents, based on similar probes in the past, will include investigators, forensics experts, crime scene technicians and a support group that will coordinate everything from transportation to security.

Ali Soufan, a former FBI special agent who led the investigation into the bombing of the USS Cole and was involved in several other high-profile terrorism cases, said that security will be a key hurdle for the team going into Libya.

“You want to solve the case, but at the same time you have to protect your people,” Soufan said.

When the agents arrive, they will have no network of informants, little to no knowledge of the area and no American law enforcement contacts with roots in the community.

Although the bureau has 60 legal attaches in offices around the globe, staffed with more than 250 agents, it did not have a “legat” in Libya. The closest legats were stationed in Cairo and Algiers.

The investigators will have to pair up with Libyan authorities to persuade witnesses to share information, photographs or video of the attacks. That partnership with local police can be tricky in an area where U.S. authorities don’t know the nature of political connections, tribal loyalties and possible sympathies with the militias, agents said.

“You can’t even begin to explain how difficult this environment is,” said Michael Rolince, the former special agent in charge of counterterrorism at the FBI’s Washington Field Office. “You’re starting from scratch.”

Once they begin working the case, the agents will also be hampered by not being able to freely travel and interview Libyans without worrying about their security.

“I’m not sure how much real investigating they can do in a place like Libya,” said retired FBI agent Daniel Coleman, an expert on al-Qaeda and the agent in charge of the Osama bin Laden case.

“The FBI can do a crime scene, but to actually go out and go into neighborhoods, we have to do that with the Libyans,” Coleman said. “Doing our job means talking to people. But you can’t just wander around Benghazi and Tripoli and interview people.”

The agents also have to establish a good working relationship with the embassy. The tension between the FBI and the U.S. Embassy in Yemen was a big obstacle in the USS Cole investigation, according to some agents, who accused then-Ambassador Barbara Bodine of blocking their work. A senior New York FBI agent and al-Qaeda expert was prevented from returning to the country in the middle of the investigation.

“It was a real travesty,” said former FBI agent Mark Rossini, the bureau’s representative on the CIA’s Counterterrorist Center who helped start the National Counterterrorism Center.

Current and former agents said the bureau does not expect to have similar problems in Libya.

“In this case, the ambassador, the direct representative of the president, was killed,” Rossini said. “I expect that once the FBI is there, they will get carte blanche from the embassy.”

Julie Tate and Greg Miller contributed to this report.