Iraqi security forces captured Sraiwi last month, they said
Iraqi security forces say they have captured leading Sunni militant Tha'ir Kadhim Sraiwi.
Mr Sraiwi is thought to be the leader of militant group Ansar al Sunna, believed to be behind a Baghdad market bombing in 2008 which killed 40 people.
Mr Sraiwi is also wanted for the killing of 17 off-duty policemen in 2006, authorities said.
There has been no independent verification of his arrest, says the BBC's Jonny Dymond, in Baghdad.
Mr Sraiwi had received support from some Iraqi politicians, though they have not been named, our correspondent said.
"We have the names of those helping this terrorist," security forces spokesman Qassim Moussawi was quoted by Reuters as saying.
"They work inside Iraq and unfortunately some of them are part of the political process. Their aim is to shake stability."
Violent attacks still take place regularly in the Iraqi capital, but the number and severity of the attacks has fallen recently.
But last week at least 35 people were killed and more than 70 were injured when a female suicide bomber attacked Shia pilgrims at a religious festival in Baghdad's Kadhamiya district.