The bodies of 20 men have been found dumped in the Iraqi capital, Baghdad.
The bodies recovered from the minibus were taken to a morgue
Eighteen were discovered in a minibus in a western district of the city populated mainly by Sunni Arabs. They appear to have been strangled or shot.
In a separate incident, two bodies were found in east Baghdad. The victims had been tied up and shot, police say.
Iraq has suffered a rise in sectarian attacks since a Shia shrine was bombed last month - but it is not clear if this was the cause in either case.
However, the dumping of bodies has previously been a sign of the violence between minority Sunni Arab and majority Shia groups.
Meanwhile gunmen in police uniforms are reported to have seized dozens of employees at a security company in the city.
The gunmen broke into the compound of the firm in the Zayouna district, Reuters news agency reported.
In other violence:
- Two policemen are killed and five people hurt in a roadside bombing in central Baghdad.
- Another roadside bomb targets a convoy of Iraqi Interior Minister Bayan Jabr in central Baghdad, killing two people, police and ministry sources say. The minister is not present.
- At least two people are killed in a car bomb in the western town of Falluja, police say.
- The US military says one of its soldiers was killed and four others wounded on Tuesday by a roadside bomb as their patrol passed in Tal Afar, north-west of Mosul.
An Iraqi security patrol found the abandoned bus on the road between Amiriyah and Khadra, two mainly Sunni districts of west Baghdad.
The men were found bound and blindfolded, officials said.
Police said the bodies had no identification papers on them. Some reports said two of the men looked like foreign Arabs.
The bodies have been taken to a morgue at the city's Yarmouk hospital.
Hospital sources told AP news agency that the deaths appeared recent and that two of the victims had been shot while the others were strangled.
The men were dressed in civilian clothes and ranged from young to middle-aged.
The bombing of the shrine at Samarra on 22 February and subsequent reprisals, in which more than 400 people have died, have sparked fears of civil war.
US ambassador to Iraq Zalmay Khalilzad said on Tuesday the country remained vulnerable to insurgents' attempts to exploit the political uncertainty caused by the bombing.
"There is a concerted effort to provoke civil war," he said.
Mr Khalilzad told the Los Angeles Times the US-led invasion in 2003 had opened "the Pandora's box" of tensions in Iraq.
US Defence Secretary Donald Rumsfeld said he did not believe a civil war was going on in Iraq, although admitted that such danger remained.
Iraq's parliament is set to sit on Sunday for the first time since the December elections.
But wrangling between Shia, Sunni Arab and Kurdish parties, particularly over the choice by the Shia-led bloc of Ibrahim Jaafari as prime minister, has cast doubt on the formation of any government of national unity.