The US military says special Iraqi Army troops have captured a senior Health Ministry official accused of being a central figure in infiltrating Shi'ite militiamen into the ministry.
A military statement did not name the official, but said he was implicated in the death of the ministry's director-general in Diyala province.
The statement said the official is suspected of funneling millions of dollars to the Mahdi Army of radical Shi'ite cleric Moqtada al-Sadr. The statement said militiamen hired by the official used ministry equipment to stage sectarian killings.
A Health Ministry spokesman named the detained official as Deputy Health Minister Hakim Zamili.
A long-awaited security crackdown is underway in Baghdad.
In the latest violence, Iraqi security officials say at least 28 people were killed in attacks across the country.
The US military said four Marines were killed on Wednesday in al-Anbar province, west of the capital.
US Major General William Caldwell says the Baghdad crackdown will be expanded as more forces arrive. The plan calls for 90-thousand Iraqi and American troops to operate in the city.
The US military is investigating Wednesday's crash of a Marine transport helicopter in al-Anbar province that killed all seven personnel aboard.
An al-Qaida-linked Sunni insurgent group says, in an Internet posting, that it shot down the helicopter.