Iraq's interior ministry has denied any involvement in the mysterious abduction of 50 security firm staff in Baghdad.
There are thought to be at least 250 security companies in Baghdad
The employees of the al-Rawafed Security Company were seized at gunpoint by people in police uniforms.
One unsubstantiated report said the attack was carried out by genuine police commandos, but others said they were militants posing as police.
The top US general in Iraq, Rick Lynch, said neither the Americans nor the Iraqis knew who had taken them.
He confirmed that gunmen, dressed like himself "in chocolate-chip desert combat uniforms", kidnapped the guards.
Meanwhile, Iraqi President Jalal Talabani has held talks with other political leaders to discuss whether to postpone the first session of the new parliament.
The assembly is required under the constitution to convene by Sunday, but the main Shia alliance has called for more time to resolve a dispute over its candidate for prime minister.
One senior Shia official told Reuters news agency that all parties had agreed the parliament must convene on Sunday, but said the alliance had not yet decided whether it would attend.
The security staff were seized from their office in the eastern Zayouna district by a large group of men who drove up in a convoy of eight four-wheel drives and pick-ups on Wednesday.
Some of the employees are said to have been officers in the Iraqi security forces during the rule of Saddam Hussein.
One unnamed senior interior ministry official was quoted by Reuters news agency as saying the police had carried out the raid after a complaint from a corporate client about the firm's services.
But the head of the police commandos, Gen Rashid Fulayh, ruled out any involvement by his troops.
"As far as we're concerned, there were no interior ministry orders to arrest these men," he told the AFP news agency.
"It's very strange that so many security guards could have been abducted without any shots being fired."
Another concern is that the abduction may have been linked to one of the many militias operating in Baghdad.
Interior ministry officials called it "a terrorist act", and the Deputy Interior Minister Ali Ghalid said it would be investigated "at the highest level".
The seizure has taken place at a time when the Interior Minister, Bayan Jabr, is calling for more control over the growing number of private security firms operating in Iraq.
There are thought to be at least 250 such companies in Baghdad, increasingly taking over from Western security contractors.
Iraq has suffered a rise in sectarian attacks since a Shia shrine was bombed last month.
The bombing of the shrine at Samarra on 22 February - and subsequent reprisals in which hundreds of people have died - have sparked fears of civil war.