BAGHDAD, Iraq (CNN) -- A top special operations officer from Lebanon's Iranian-backed militia Hezbollah has been captured in Iraq, where U.S. officials say he played a key role in a January attack that killed five Americans.
A 2006 poster in Iraq shows Shiite cleric Muqtada al-Sadr, left, and Lebanese Hezbollah leader Hassan Nasrallah.
Ali Mussa Daqduq, an explosives expert, was captured in March in the southern Iraqi city of Basra, where he was helping train and lead Shiite militias fighting coalition troops, U.S. intelligence officials told CNN.
Daqduq pretended to be deaf and mute when captured, and his identity was not known for weeks, the officials said.
Once uncovered, however, they said he began to talk, and they now believe he played a crucial role in the January 20 attack in Karbala. Watch Michael Ware's report on Daqduq »
Hezbollah fought Israeli troops in a month-long war in southern Lebanon in 2006, a conflict sparked by a cross-border raid in which Hezbollah fighters killed three Israeli soldiers and took two others captive. The conflict ended with a U.N.-brokered cease-fire, and the Israeli soldiers remained captive when the fighting ended.
Intelligence officials say Daqduq is one of Hezbollah's top special operations commanders, an expert in the use of roadside bombs. The Americans say he, along with the Iraqi militia commanders he worked with, has admitted working with Iran's elite Quds Force special operations unit.
U.S. commanders have said for months that Iraqi militants have been receiving weapons and training from members of the Quds Force, an element of Iran's Revolutionary Guards. Washington has demanded the Iranian government stop the flow of arms and militants across its border -- which, along with the diplomatic standoff over Iran's nuclear fuel program, has raised fears of a wider war in the region.
Iran, which has close ties to the Shiite parties that control Iraqi Prime Minister Nuri al-Maliki's government, has repeatedly denied the allegations. But U.S. intelligence officials said the Quds Force has been backing the creation of Shiite "special groups" modeled on Hezbollah, which holds sway over much of southern Lebanon.
The U.S. military declined official comment on Daqduq's arrest, as did the Iraqi government. But documents and forensic evidence, seen by members of the Iraqi government and shown to CNN, support the claims.
Senior U.S. intelligence officials said Daqduq was captured in a raid aimed at seizing another Shiite militant leader suspected of involvement in the January 20 attack in Karbala.
U.S. sources and Iraqi militia sources have said the carefully planned operation was meant to take captives who could be traded for five Iranians held by U.S. troops since a January 10 raid in Irbil, in northern Iraq. But the Karbala attack went awry, resulting in the deaths of the five Americans.
Qais Khazali, a onetime spokesman for anti-American cleric Muqtada al-Sadr's Mehdi Army, was one of the men sought by American troops in connection with the attack. By the time of his March arrest, he had left the Mehdi Army and was leading one of the "special groups," according to U.S. intelligence.
In searching for Khazali, U.S. and allied troops found computer documents detailing the planning, training and conduct of the failed kidnapping. And they found Daqduq, whom intelligence officials said has admitted working on behalf of Iran.
Contacted by CNN, a Hezbollah spokesman in Lebanon said he would not dignify the U.S. allegations with a response. And it remains unclear why Hezbollah's leadership would risk sending advisers to Iraq: American intelligence officers suspect Hezbollah -- which is indebted to Iran for decades of military and financial support -- had no choice.
Meanwhile, representatives of the Mehdi Army deny receiving any military aid, though they say they share some of Hezbollah's ideals.
"I say clearly that we do not accept any logistic, financial, or any other kind of support from anyone outside the borders of Iraq," said Rassim al-Marwani, Sadr's cultural adviser. E-mail to a friend
CNN's Thomas Evans contributed to this report.