(CNN) -- Several Yemeni men belonging to al Qaeda took part in the terrorist attack on the U.S. diplomatic compound in Benghazi last September, according to several sources who have spoken with CNN.
One senior U.S. law enforcement official told CNN that "three or four members of al Qaeda in the Arabian Peninsula," or AQAP, took part in the attack.
Another source briefed on the Benghazi investigation said Western intelligence services suspect the men may have been sent by the group specifically to carry out the attack. But it's not been ruled out that they were already in the city and participated as the opportunity arose.
The attack on the compound and subsequently on a "safe-house" to which Americans had been evacuated left four U.S. citizens dead, including the ambassador to Libya, Chris Stevens.
If the AQAP members were dispatched to Benghazi, it would be further evidence of a new level of co-operation among jihadist groups throughout the Middle East and North Africa, counterterrorism analysts say.
According to one source, counterterrorism officials learned the identity of the men and established they had spent two nights in Benghazi after the attack. Western intelligence agencies began trying to track the men in the aftermath of the terrorist attack, but were always behind in their manhunt.
They were later traced to northern Mali, where they are believed to have connected with a fighting group commanded by Moktar Belmoktar, a prominent jihadist leader, according to a senior law enforcement source.
The trail appears to have then gone cold. In early 2013, jihadists were driven out of many areas of northern Mali in a French-led offensive.
Another source briefed on the investigation had previously told CNN that Belmoktar had received a call in the aftermath of the Benghazi attack from someone in or close to the city. Whoever made the call was excited.
"Mabruk, Mabruk!" he repeated, meaning "Congratulations" in Arabic.
There is no proof the call was specifically about the attack, but the source says that is the assumption among those with knowledge of the call. One source says the phone call was discovered when a Western intelligence service trawled through intercepts of communications made in the wake of the attack.
CIA officials told CNN they had no comment on whether any call had been intercepted.
One other source briefed by Western intelligence told CNN a call was intercepted but said only that it was placed to an AQIM commander, not specifically Belmoktar.
Belmoktar is an Algerian terrorist operative linked to al Qaeda in the Islamic Maghreb who claimed responsibility for the attack on the In Amenas gas facility in southern Algeria in January this year. Some 38 people were killed during a three-day siege there.
Chadian troops supporting the French intervention in Mali claimed in March that Belmoktar and others in his group had been killed during an operation in the remote Adrar des Ifhogas mountain range.
There has never been any confirmation of his death, and one source briefed by Western and regional intelligence officials told CNN that Belmoktar may have started operating in the "desert triangle" straddling the borders of Algeria, Niger and Libya.
Belmoktar is believed to have moved to the region in late 2012 after signs that an international intervention in Mali was growing more likely. Known as 'the Salvador pass,' the area is a key transit points for drug traffickers and international criminal groups.
The FBI released grainy photographs Wednesday of three men said to have been at the Benghazi compound on the night of the attack, saying it was "seeking information" on them. It is not known whether any of them are the AQAP individuals.
AQAP is regarded as one of the most active and dangerous of al Qaeda franchises. It has tried to launch several attacks on the U.S. homeland. On Christmas Day 2009, a Nigerian recruited by the group attempted to blow up a plane flying into Detroit but failed because his device malfunctioned. The following October the group attempted to blow up planes heading to the United States with printer bombs disguised as air cargo. The packages were intercepted after a tip from Saudi intelligence. And in April 2012, a British informant working for Saudi counterterrorism thwarted a new plot by the group to bomb a U.S-bound airliner. The informant had infiltrated the group and was selected by them to launch the attack.
AQAP was the first al Qaeda affiliate to comment on the Benghazi attack. On September 14 it released a statement arguing the attack was revenge for the death of Abu Yahya al-Libi, a senior al Qaeda operative, in Pakistan in June 2012. It did not claim responsibility for the attack.
On September 10 -- at least 18 hours before the attack -- al Qaeda leader Ayman al-Zawahiri, in a video timed for the anniversary of 9/11, called for attacks on Americans in Libya to avenge the death of al-Libi.
In March, Libyan authorities detained a man called Faraj al-Shibli in Libya on suspicion of links to the attack, according to several officials. The FBI was able to interview him in the presence of Libyan officials, according to one Libyan source. It appears al-Shibli was detained after returning from a trip to Pakistan, sources said.
It remains unclear exactly whether al-Shibli was present at the U.S. compound at the time of the attack. It's also unclear whether his detention is likely to lead to charges in connection with the attack. Investigators have learned that al-Shibli has had contact with the Yemen-based al Qaeda in the Arabian Peninsula as well as al Qaeda members in Pakistan, sources said.
The Libyan Interior Ministry refused to confirm he is still in custody, saying it could not comment on an ongoing investigation.
Al-Shibli is the only known suspect in custody in connection with the attack in Benghazi. A 26-year-old Tunisian, Ali Ani al-Harzi, was held in Tunis for several weeks in connection with the assault on the compound after being extradited from Turkey. But he was released by a Tunisian judge in January on grounds on insufficient evidence.
In December, a U.S. official with direct knowledge of the investigation said authorities were examining whether the alleged leader of a post-revolution terrorist network in Egypt had played a role in the September 11 attack. Mohammed Jamal Abu Ahmed was released from jail after the downfall of Egyptian President Hosni Mubarak and is believed to be the driving force behind a new militant group, according to two U.S. officials.
Abu Ahmed was previously a member of al-Zawahiri's group, Egyptian Islamic Jihad. He is currently in prison in Egypt after being arrested in December, when police raided a Cairo apartment allegedly being used by a jihadist group. An associate of Abu Ahmed's subsequently said that he had not been in Benghazi or anywhere in Libya on the day of the attack on the compound.
In the wake of the revolts that have shaken the Arab world, al Qaeda sympathizers have found new space in which to operate, and would-be jihadists have found new causes to embrace. In Syria, the al-Nusra Front has proclaimed its links to the al Qaeda affiliate in Iraq. Militants from Tunisia, Libya and elsewhere have gone to Syria. In west Africa, Nigerian jihadists with Boko Haram have established links with al Qaeda in the Islamic Maghreb and trained in Mali.
If AQAP sent members thousands of miles to help launch an attack on U.S. diplomats, it would show that even if al Qaeda central remains under pressure, its fellow travelers are finding new ways to continue its campaign of terror.
CNN's Jomana Karadsheh contributed to this report.