Officials in the US say they have identified five men who might be responsible for the attack on the diplomatic mission in Benghazi, Libya, in 2012 and there is enough evidence to justify seizing them by military force as suspected terrorists or killing them with a drone strike. However, there is not 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. The investigation has been slowed by the reduced US intelligence presence in the region since the 11 September 2012 attacks and 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 the Gaddafi regime.

The decision not to seize the men militarily underscores Washington's desire to move away from hunting terrorists as enemy combatants and holding them at the military prison in Guantánamo Bay, Cuba. The preference is towards 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 co-operation, and tried in the US criminal justice system.

A senior Obama administration official, speaking to the Associated Press on condition of anonymity, said the FBI had identified a number of individuals it believed could have information or may have been involved and was considering options to apprehend them. But taking action in remote eastern Libya would be difficult and America's relationship with Libya had to be considered.

The Libyan embassy did not respond to multiple requests for comment, the AP said.

The FBI and other US intelligence agencies identified the men through contacts in Libya and by monitoring their communications, officials said. 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 attack.

US officials say FBI surveillance has gathered proof that the five men were either at the scene of the first attack or somehow involved. In intercepts at least one of them bragged about taking part. Some of the men had also been in contact with a network of well-known regional jihadists, including al-Qaida in the Islamic Maghreb, the officials said.

The attack on the US diplomatic mission killed the 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, criticising 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 minimise its effect on the president's campaign.

The FBI released photos of three of the five suspects earlier this month. 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.