Iraq has launched an investigation into claims by the US military that an Iraqi interior ministry "death squad" has been targeting Sunni Arab Iraqis.
Sunni Arab Iraqis have long complained about death squads
The probe comes after a US general revealed the arrest of 22 policemen allegedly on a mission to kill a Sunni.
"We have found one of the death squads. They are part of the police force," US Maj Gen Joseph Peterson said.
Sunnis have long accused Iraqi forces of operating death squads - but the claims have never been substantiated.
Iraqi deputy interior minister Maj Gen Hussein Kamal said his ministry had set up an inquiry.
"The interior minister has formed an investigation committee to learn more about the Sunni person and those 22 men, particularly whether they work for the interior ministry or claim to belong to the ministry," he told the Associated Press news agency.
Hundreds of Sunni Arab Iraqis have been found dead since the 2003 war in what appear to have been extra-judicial killings.
On Wednesday, the bodies of four unidentified men were found in Baghdad's Shia district of Shula. They had been handcuffed, blindfolded and shot in the head.
Iraqi insurgents have also often used a similar tactic against Iraqis working with international forces or the Iraqi government.
Gen Peterson, who is in charge of training the Iraqi police, told the Chicago Tribune on Wednesday that US forces had stumbled across the first evidence of death squads within the interior ministry.
The 22 interior ministry traffic policemen, dressed in police commando uniforms, were arrested in late January at an Iraqi army checkpoint in northern Baghdad and asked what they were doing.
They told soldiers they were taking a Sunni man away to be shot dead.
"The amazing thing is... they tell you exactly what they're going to do," Gen Peterson said.
Gen Peterson said US forces were holding four of the men at the Abu Ghraib prison and that the 18 other men were being detained at an Iraqi jail.
The Sunni man, who was accused of murder, is also being detained.
Subsequent investigations found the four men in US custody are linked to the Badr Organisation, the armed militia of one of Iraq's main Shia parties, the Supreme Council for the Islamic Revolution in Iraq.
But Gen Peterson said he was convinced Iraqi Interior Minster Bayan Jabr, a member of Sciri, had no knowledge of or involvement in the death squads.
"Who are these guys? That's what the minister is trying to find out," he said.
"They are discrediting him and his organisation. He wants to find these guys. He does not support them."
But Gen Peterson said he believed other death squads were operating within the Iraqi security forces.
"It's an issue of loyalties, of allegiance," he said. "If you're still wearing your Badr T-shirt under your uniform, that's a problem."
Iraqi Human Rights Minister Narmin Uthman said she believed lower-level officials were helping the death squads.
"These officials are helping the criminals by informing them on where targeted people are going or where people are living," she told AP.
A spokesman for the country's main Sunni Arab party, the Iraqi Islamic Party, backed the launch of the investigation.
"For a very long time we have been talking about such violations and we have been telling the interior ministry officials that there are squads that raid houses and arrest people who are found later executed in different parts of the capital," Nasser al-Ani said.