BAGHDAD — A senior official in Nouri al-Maliki's government was in custody Thursday suspected of ties to Iranian-backed Shiite militias and plotting a June bombing that killed 10 people, including four Americans, Iraqi authorities said.

The arrest of Ali al-Lami — taken Wednesday as he left a plane arriving from Lebanon — reinforced suspicions about Tehran's influence within the Shiite-led Iraqi government and could open wider probes into Shiite networks, including possible links to Lebanon's Hezbollah.

Al-Lami heads a commission responsible for keeping Saddam Hussein loyalists out of government posts and has been a target of criticism from Sunni leaders who claim the government wants to limit the overall Sunni voice in political and security issues.

He was arrested by U.S. and Iraqi troops at Baghdad's airport as he returned with his family from medical treatment in Beirut, said a member of his committee, Qaiser Watout.

U.S. and Iraqi troops were waiting for al-Lami as the plane's doors opened, Watout said.

"We condemn this act," Watout said. "Al-Lami was a moderate official and we are surprised by his arrest."

U.S. military officials would not confirm the arrest of al-Lami, who has been involved in government affairs since shortly after Saddam's fall in 2003.

But the U.S. command said a "suspected senior" leader of Iranian-backed "Special Groups" militias was detained at the airport for allegedly planning the June 24 bombing of a municipal building in the capital's Shiite district of Sadr City. Two American soldiers and two State Department employees died in the blast along with six Iraqis.

"The man has been known to travel in and out of Iraq to neighboring nations including Iran and Lebanon, where it is believed he meets and helps run the Iranian-backed Special Groups in Iraq," the U.S. military statement said.

In Washington, a senior U.S. military intelligence official said Thursday that the statement referred to al-Lami and that he was believed to have information that would lead investigators to people connected to "other countries," an apparent reference to Iran and Lebanon.

The official spoke on condition of anonymity because of the sensitivity of the case but added he had personally reviewed "multiple and corroborating reports" pointing to al-Lami.

Iraqi Shiite parties that dominate the government maintain close ties to Iran, where many key figures spent years in exile during Saddam's rule. U.S. officials have long maintained that Iran's Revolutionary Guards, through its elite Quds Force, arms and trains Shiite extremists — a charge Tehran denies.