Iraq will deploy thousands of police to prevent Sunni militants from bombing a major Shiite ceremony next week, while the bullet-riddled bodies of two Shiites were found Sunday in the latest round of killings between rival Sunni and Shiite groups.

The U.S. military, meanwhile, announced the release of about 50 Iraqi detainees, but no women were among them. The freeing of women is a demand by kidnappers of American journalist Jill Carroll, who was abducted Jan. 7 in Baghdad.

Sectarian anxieties are running high in Iraq amid a spate of killings and kidnappings. The violence threatens talks to form a national unity government, which the U.S. hopes will bring the country's main Shiite, Sunni Arab and Kurdish parties closer together and end the bloodshed.

Three gunmen shot dead two policemen in the northern city of Kirkuk, and a roadside bomb killed two civilians in Madain, about 14 miles southeast of Baghdad, police said. Five civilians and two policemen were also wounded in the Madain blast, which targeted a passing police convoy.

Amid the tensions, Iraqi forces are stepping up security ahead of the most important date in the Shiite calendar — the feast of Ashoura — to prevent a repeat of homicide bombings by Sunni extremists that killed at least 230 people during the ceremonies in the past two years.

Ashoura, which falls this year on Thursday, marks the 7th century death of the revered Shiite saint Imam Hussein, grandson of Islam's Prophet Muhammad. Hundreds of thousands of Iraqi Shiites are expected to take part in ceremonies, including self flagellation, in a show of grief to mark Hussein's death in battle.

Ceremonies have already begun but reach their climax Thursday.

Interior Ministry undersecretary Maj. Gen. Ahmed al-Khafaji said Iraqi police forces will be on high alert to prevent a repeat of previous Ashoura-linked terror attacks claimed by Al Qaeda in Iraq, a group headed by Jordanian-born militant Abu Musab al-Zarqawi.

"The attacks that happened on Ashoura during the previous years gave us more insight about what measures we should take to prevent the repetition of the same attacks," al-Khafaji told The Associated Press.

Extra checkpoints and police patrols will be operating in major Shiite centers such as the holy southern cities of Karbala and Najaf and predominantly Shiite areas in Baghdad such as the northern suburb of Kazimiyah.

At least 8,000 police will be on duty in Karbala, where security authorities have warned hotel owners not to accept any non-Iraqi Arabs or Iraqis without identity documents and to inform authorities about suspicious individuals, said police spokesman Abdul Rahman Mishawi.

Ashoura falls on the 10th day of Muharram in the Islamic calendar and is the holiest festival for Shiites, who comprise 60 percent of Iraq's 27 million people.

It marks the anniversary in the Islamic lunar calendar of the death of Imam Hussein, who was killed in 680 A.D. during a battle in Karbala for leadership of the faith. Hussein's death caused the split in Islam between Shiites and Sunni Muslims.

Under Saddam Hussein, Shiites were restricted from performing Ashoura-related rituals, such as processions of men beating themselves with their hands, chains and the flat edges of swords in shows of grief.

Shiites resumed practice of the rites after Saddam's ouster, with rituals often turning into frenzied, blood-soaked outpourings of religious devotion.

Sunday's discovery of the two Shiite men came two days after 14 Sunni Arab men were found shot in a truck parked on the northern edge of the capital.

Police Lt. Thaer Mahmoud said the slain Shiites were found on the highway linking Baghdad's northern suburb of Shula to Taji, a town 12 miles north of Baghdad. Both bodies were bound and wearing black clothes in apparent preparation for Ashoura, which is expected to begin Wednesday.

"We suspect that the victims are Shiites who were participating in Ashoura services because they were wearing black clothes and were killed by Sunni extremists and terrorists to enflame sectarian tensions," Mahmoud said.

Sunni Arab leaders have condemned the killings of the 14 men and threatened to call for a nationwide campaign of civil disobedience if Interior Minister Bayan Jabr, a prominent Shiite, is not fired.

Sunnis accuse Jabr of directing security forces to attack members of their community, but the minister denies such claims.

Armed Sunnis and Shiites have been detaining and killing members of each other's communities in a campaign of attacks that erupted after the U.S.-led invasion of Iraq in 2003.

The Shiites have taken control of security forces and major political positions in the country since the ouster of Saddam Hussein, who oppressed the country's majority Shiites in favor of Sunni Arabs.

Iraq's raging insurgency, which includes kidnappings, bombings and assassinations, is believed to be fanned mostly by disgruntled Sunnis.