BAGHDAD, Iraq, May 26 - No one knows who tortured and killed Hassan al-Nuaimi, a Sunni Arab cleric whose body was found in an empty lot here last week, with a hole drilled in his head and both eyes missing. But the various theories have a distinctly sectarian tinge.
The Shiite police chief investigating the death said he suspected Sunni Arab extremists who have driven much of the insurgency in Iraq, much of it aimed at Shiites. The Sunni family mourning the cleric pointed the finger at the Badr Organization, a Shiite militia. But with Mr. Nuaimi buried, the truth, as so often with killings in Iraq, seems to be lost in rumor and allegations.
The only sure thing is that Mr. Nuaimi and another Sunni man who helped write sermons were killed within 12 hours of their disappearance from a mixed Sunni-Shiite neighborhood in northeast Baghdad.
Their deaths, amid violence that has taken more than 550 lives across Iraq this month, renewed concern that the bloodshed may be shifting ever more toward crudely sectarian killings.
Hard-line Sunni leaders have pressed the case. "The killing in Iraq now is according to religious identity," said Sheik Abdel Nasir al-Janabi, a religious Sunni and a hard-line member of the National Dialogue Council, a Sunni political group that claims to have ties to the insurgency. "Now you're killed because you're a Sunni Arab."
Shiite leaders, including Grand Ayatollah Ali al-Sistani, Iraq's most powerful Shiite cleric, have responded to such talk with calls for calm and renewed appeals to Shiites that they place their trust in Iraq's fledgling democracy, not revenge killings.
But the urgency of the Shiite leaders' appeals reflects a deepening fear that the welter of allegations about Shiite death squads going after Sunni Arabs, true or false, may create a new reality, prompting still more sectarian killings and pushing the country ever closer to the brink of civil war.
"We are drifting into a sectarian society," said Ghassan al-Atiyya, a secular Shiite and the director of the Iraqi Foundation for Development and Democracy, a Baghdad research institute. "The Americans, instead of strengthening liberal and secular, they are now hostage of Sciri," he said, referring to a religious Shiite political group, "and Kurds."
"They let the genie out of the bottle," Mr. Atiyya said.
Iraq's Shiite majority were the main victims of Saddam Hussein's repression and have been among the principal targets of the insurgents. On Monday insurgents killed at least 33 Shiites in three car bomb attacks in Iraq, and on Thursday two members of Shiite political parties were assassinated.
For the past year Shiites have been attacked at mosques, weddings, funerals and crowded marketplaces. Abu Musab al-Zarqawi, the most ruthless insurgent leader, has urged still more killing, calling Shiites "apostates" and usurpers of the Sunni Arab primacy in Iraq that ended with the overthrow of Mr. Hussein. On Wednesday his group boasted of killing Shiites in the northern city of Tal Afar.
But when Iraq got its first-ever Shiite majority government three weeks ago, the transition was accompanied by a new wave of terror that included attacks on Sunni Arab leaders, including clerics, and even fruit and vegetable sellers. Sunni leaders have blamed Shiite militias that they say work behind the scenes with official army and police forces, a charge that Shiites deny.
Prime Minister Ibrahim al-Jaafari suggested Thursday that there might be some truth in Sunni allegations of Shiite death squads. "I am alarmed," Dr. Jaafari said. "We will act very strongly against those who take the law into their own hands."
Sunni leaders have accused Shiite-led security forces of raiding mosques, arresting more than 300 Sunni clerics and worshipers, and killing several of them, including Mr. Nuaimi. His family has said he was taken from his home by men wearing Iraqi security force uniforms.
On Monday the Iraqi Islamic Party, a Sunni political group, condemned several sets of killings that it said had been carried out by government forces.
Sheik Khalaf al-Aliyan, a member of the National Dialogue Council, a coalition of Sunni political leaders, said he had evidence that Shiite political parties had drawn up a list of 4,000 Sunnis they intended to assassinate, a charge that Shiite leaders have dismissed as preposterous.
"We are approaching the red line," said Saleh Mutlak, a moderate member of the council, which has also urged Sunni participation in the political process.
Most Iraqis, whether Shiite or Sunni, Arab or Kurd, Muslim or Christian, have held tightly to a legend about the Iraqi past. Iraqis, they say, have never defined themselves primarily by religion or ethnic origin but have submerged themselves in a common identity as Iraqis. Even now, reporters who ask people which community they belong to tend to get a common answer. "I am Iraqi," men and women will say, or, with equal insistence, "I am a Muslim."
Even so, in the last two years a strengthened sense of religious and ethnic identity began to course through Iraqi Shiite and Kurdish communities, which had endured the most repression under Mr. Hussein. Moderate Sunnis worry that the newfound identity combined with Shiites' and Kurds' new positions of power may deepen sectarian rifts.
"I came back to Iraq with the assumption that these religious and sectarian tendencies were not that strong," said Adnan Pachachi, 81, a Sunni statesman and former foreign minister, who returned from exile in 2003 and became one of the most trusted advisers of the American occupation authorities. "But in times of trouble people tend to go toward religion, and the religious parties make use of that very skillfully."
The shift in power has been a major irritant. Shiites, for years a downtrodden underclass, took power in elections in January and now control Iraq's government, Parliament and much of its police and security forces. Sunni Arabs, who ran Iraq from the time of the Ottoman Empire, are chafing under that rule but have little leverage after boycotting the elections.
That has left Sunni Arabs with a bitterness that leaves them more open to hard-liners' appeals.
"There's a sense of alienation and embitterment among Sunnis," said Hassan al-Bazzaz, an international relations professor at Baghdad University and director of a center that studies public opinion. "Maybe that's why a lot of them are ready to share suspicions about what's happening."
A sampling of opinions at Friday Prayer at Baghdad's main hard-line Sunni mosque, called the Mother of All Battles, showed sharp Sunni Arab anger at Shiites, while Shiites near Al Mohsen Mosque in Sadr City, a Shiite district, expressed little or no anger.
For Sunnis, perhaps the strongest symbol of Shiite bullying is the Shiite militia called the Badr Brigade. The group was formed in the 1980's in Iran as a fighting force of Iraqi Shiites opposed to Mr. Hussein. Many of its members returned after the American invasion. The group was ordered to disband but still exists informally, with a former member now running the Interior Ministry.
Saad Qindeel, the head of the political bureau of the party affiliated with the group, said it had put down its weapons and become a civil organization. But an American official and Iraqi officials say the group is used to gather intelligence.
There was evidence of that in at least one set of arrests two weeks ago, when according to a Shiite commander of an army commando unit called the Wolf Brigade, Badr intelligence pointed out three Palestinians and an Iraqi who they said had carried out a suicide bombing in Baghdad.
"They are very active," Mr. Pachachi said of the Badr Brigade. "This is the fear that many Sunnis have, that the Badr Brigade will be using the great advantage of being in the government to strengthen their position and try to defeat their opponents."
And small signs of sectarian strife have surfaced in Baghdad neighborhoods.
In a mysterious incident earlier this month, a Shiite student at the Baghdad University College of Pharmacy was killed after arguing with a Sunni dean over a rally he wanted to hold to celebrate the new government. Protests ensued, and the college was closed for several days.
A rash of five or six arrests of Sunni businessmen in the last two weeks has raised concerns among wealthy Sunnis that a campaign is being waged against them. "Anybody who is Sunni and has money is a target," said one member of the Iraqi Bankers Association, who declined to give his name out of fear that he would be arrested. "This is a witch hunt."
Shiite leaders, now in power, have called for restraint despite the killings of Shiites. Even Moktada alSadr, a notoriously rebellious cleric who led an uprising against Americans last summer, offered himself as a mediator on Sunday night.
At the funeral of a Shiite cleric who was gunned down in his car last week, the words of Abdel Karim al-Jazaery, a close friend of the cleric, were typical.
"The point is to make Muslims two parts, to divide the good people, Sunni and Shiite," Mr. Jazaery said of the killers. "God willing, they won't succeed."
Outside the funeral tent, members of the Wolf Brigade stood guard.