The arrest of Ali al-Lami could bring serious pressure on the U.S.-backed government from Sunni groups and others worried about Iranian and Shiite militia influence in the top ranks of the Iraqi leadership.
The U.S. military confirmed it arrested a senior Shiite figure at the airport Wednesday, but would not release the name or say whether it was al-Lami.
The military said the man detained is believed to be a senior leader of Iranian-backed militias and behind a June bomb blast in Baghdad that killed four Americans and six Iraqis.
Al-Lami heads a committee that is in charge of keeping senior Saddam Hussein loyalists out of government positions. He was returning to Iraq from Lebanon where he sought medical treatment when he was detained, said Qaiser Watout, a member of al-Lami's committee.
U.S. forces were waiting for al-Lami as the plane's doors opened, said Watout, adding that al-Lami's family was allowed to proceed. "We condemn this act," Watout said. "Al-Lami was a moderate official and we are surprised by his arrest."
U.S. military officials said the man arrested at the airport is believed to be a senior leader of "special groups" - Iranian-backed militiamen in Iraq.
The military said the man, who was known to travel to Iran and Lebanon, was detained after his plane landed at the airport.
Two U.S. soldiers and two American civilians were among the 10 killed in the June 24 bombing inside a district council building in Baghdad's Shiite slum of Sadr City.
Iraqi officials have said it appeared to be an inside job and suspicion at the time fell on the headquarters' Shiite Muslim guard force.
The Iraqi Defense Ministry also said it appeared the Iraqi council members and not the Americans were the main target of the blast, which came ahead of an election to choose a new chairman of the council.
The deadly attack came as the U.S. military and civilian officials were stepping up efforts to take advantage of a sharp drop in violence to promote the local administration and restore services in Sadr City and other areas.
Meanwhile, anti-American Shiite cleric Muqtada al-Sadr released a statement saying his largely disbanded Mahdi Army militia would extend its cease-fire "until further notice."
The statement, which was read by an aide in the Iraqi holy city of Najaf, also warned that any "person who violates" the truce would no longer be considered part of the Mahdi Army.
Al-Sadr's militia battled U.S. and Iraqi forces for years, but last year, he declared a cease-fire. The truce had been extended in six-month increments, but Thursday's statement indicated it would now be considered open-ended.
Last month, al-Sadr, who lives in Iran but retains significant clout in Iraq, announced he was transforming his militia into a social welfare body with a few guerrilla cells to attack U.S. troops if Washington doesn't agree to leave Iraq. The announcement followed setbacks in battles with the U.S.-supported Iraqi army in Baghdad, Basra and Amarah.
In other developments: