The US has identified five men who might be responsible for the attack on the diplomatic mission in Benghazi, Libya, last year, and has enough evidence to justify seizing them by military force as suspected terrorists, officials say.
But there isn't enough proof to try them in a US civilian court as the Obama administration prefers.
The men remain at large while the FBI gathers evidence.
But the investigation has been slowed by the reduced US intelligence presence in the region since the Sept 11, 2012, attacks, and by the limited ability to assist by Libya's post-revolutionary law enforcement and intelligence agencies, which are still in their infancy since the overthrow of dictator Muammar Gaddafi.
The decision not to seize the men militarily underscores the White House aim to move away from hunting terrorists as enemy combatants and holding them at the military prison in Guantanamo Bay, Cuba.
The preference is toward a process in which most are apprehended and tried by the countries where they are living or arrested by the US with the host country's cooperation and tried in the US criminal justice system.
Using military force to detain the men might also harm fledgling relations with Libya and other post-Arab-Spring governments with whom the US is trying to build partnerships to hunt al Qaeda as the organization expands throughout the region.
A senior administration official said the FBI has identified a number of individuals that it believes have information or may have been involved, and is considering options to bring those responsible to justice.
But taking action in remote eastern Libya would be difficult. America's relationship with Libya would be weighed as part of those options, the official said, speaking only on condition of anonymity because the official was not authorized to discuss the effort publicly.
The Libyan Embassy did not respond to multiple requests for comment.
Waiting to prosecute suspects instead of grabbing them now could add to the political weight the Benghazi case already carries. The attack on the US diplomatic mission killed ambassador Chris Stevens and three other Americans weeks before President Barack Obama's re-election.
Since then, Republicans in Congress have condemned the administration's handling of the situation, criticizing the level of embassy security, questioning the talking points provided to UN ambassador Susan Rice for her public appearances to explain the attack and suggesting the White House tried to play down the incident to minimize its effect on the president's campaign.
The FBI released photos of three of the five suspects earlier this month, asking the public to provide more information on the men pictured.
The images were captured by security cameras at the US diplomatic post during the attack, but it took weeks for the FBI to see and study them.
It took the agency three weeks to get to Libya because of security problems, so Libyan officials had to get the cameras and send them to US officials in Tripoli, the capital.
The FBI and other US intelligence agencies identified the men through contacts in Libya and by monitoring their communications. They are thought to be members of Ansar al-Shariah, the Libyan militia group whose fighters were seen near the US diplomatic facility prior to the violence.
Republican lawmakers continue to call for the Obama administration to provide more information about the attack.
The White House released 99 pages of emails about the talking points drafted by the intelligence community that Rice used to describe the attack, initially suggesting they were part of a series of regional protests about an anti-Islamic film.
In those emails, administration officials agreed to remove from the talking points all mentions of terror groups such as Ansar al-Shariah or al Qaeda, because the intelligence pointing to those groups' involvement was still unclear and because some officials didn't want to give Congress ammunition to criticize the administration.
US officials say the FBI has proof that the five men were either at the scene of the first attack or somehow involved because of intercepts of at least one of them bragging about taking part.
Some of the men have also been in contact with a network of well-known regional Jihadists, including al Qaeda in the Islamic Maghreb.
The US has decided that the evidence it has now would be enough for a military operation to seize the men for questioning, but not enough for a civilian arrest or a drone strike against them, the officials said.
The US has kept them under surveillance, mostly by electronic means. There was a worry that the men could get spooked and hide, but so far, not even the FBI's release of surveillance video stills has done that.
FBI investigators are hoping for more evidence, such as other video of the attack that might show the suspects in the act of setting the fires that ultimately killed the ambassador and his communications specialist, or firing the mortars hours later at the CIA base where the surviving diplomats took shelter - or a Libyan witness willing to testify against the suspects in a US courtroom.
But Rep Howard P "Buck" McKeon, the Republican chairman of the House Armed Services Committee, said he is concerned the Obama administration is treating terrorism as criminal actions instead of acts of war that would elicit a much harsher response from the United States.