Updated

U.S. forces in Iraq believe they may be facing an Al Qaeda cell in Fallujah after two men with suspected ties to the terror network were captured in the last week, sources told Fox News Friday.

Husam al-Yemeni was arrested by U.S. forces last Thursday and is said to be part of the leadership structure of Ansar al-Islam (search), the Al Qaeda-associated terrorist group based in Iraqi Kurdistan. Some U.S. officials described al-Yemeni as the first Al Qaeda operative captured in Iraq.

Another possible Al Qaeda operative, Hasan Ghul, was detained Thursday in Iraq. Ghul, a Pakistani, is known to have been an Al Qaeda member since the early 1990s, when Al Qaeda was established.

Officials said it was too early to be sure, but at least one guerrilla cell in Fallujah (search) — a Sunni Arab city known for its fierce enmity toward American forces — was believed to be linked to Al Qaeda. The officials said three other possible Al Qaeda operatives — two Egyptians and an Iraqi — had been captured in raids Sunday.

U.S. commanders in Iraq and Pentagon officials had said this week they were seeing signs foreign fighters were attempting to organize both inside and outside Iraq.

One official said Ghul was "definitely in Iraq to promote an Al Qaeda, Islamic extremist agenda." Ghul is described by officials as a facilitator known in terrorist circles as "the Gatekeeper" who moves money and people around the Middle East, Africa and possibly beyond. Officials added that Ghul has extensive contacts in Al Qaeda and wider terrorist communities, and is thought to have had some kind of connection to the 1998 East African embassy bombings, though officials stress those links are still being probed.

Al-Yemeni is believed to be the right-hand man to Abu Zarqawi (search), a man the Bush administration says has worked directly with Usama bin Laden. Zarqawi, who is believed to have been operating in Iraq before March's invasion, is still at large.

Early last year, U.S. Secretary of State Colin Powell detailed Zarqawi's significance in an appearance before the U.N. Security Council.

"Iraq today harbors a deadly terrorist network headed by Abu Musab Zarqawi, an associate and collaborator of Usama bin Laden and his Al Qaeda lieutenants," Powell said.

Powell described Zarqawi as a Palestinian born in Jordan who fought in Afghanistan more than a decade ago. In 2000, Zarqawi returned to Afghanistan, where Powell said he oversaw terrorist training camps.

"One of his specialties at the camp was poisons," Powell said. "When our coalition ousted the Taliban, the Zarqawi network helped establish another poison and explosives training center."

Fox News' Bret Baier and Ian McCaleb contributed to this report.