Baghdad, Iraq ? Shiite clerics joined Sunni preachers in a march of thousands of mostly black-clad men Wednesday, trying to keep sectarian passions in check after a horrific attack on Shiite pilgrims that raised fears of civil war.

U.S. and Iraqi officials disagreed over how many people died in Tuesday’s bombings in Baghdad and Karbala, the deadliest here since the fall of Saddam Hussein. The Iraqi Governing Council said 271 people were killed. U.S. officials put the toll at 117.

The attacks, at some of the holiest shrines of Shiite Islam and on the most sacred day in the Shiite calendar, threatened to turn Shiites against Sunnis if the bombers were found to have been Iraqi Sunni extremists.

But strife with the country’s Sunni minority would hardly be in the interests of the Shiites, who stand on the verge of achieving their dream of real political power after generations of suppression. Civil war would threaten those dreams, and the community’s influential clergy appeared eager to keep passions in check.

No group claimed responsibility for Tuesday’s attacks. However, the top U.S. commander in the Middle East, Gen. John Abizaid, said Wednesday the United States has evidence that al-Qaida-linked Jordanian militant Abu Musab al-Zarqawi was behind the bombings.

U.S. officials said 15 people were detained in Karbala in connection with the attacks, though none was charged. Among those detained were five Farsi speakers, a suggestion that they were Iranians. About 100,000 Iranians were believed to have come to Iraq for the Ashoura religious rituals, and Iran’s news agency said 23 Iranians were among the dead.

In what appeared to be a nod to criticism from Iraq’s top Shiite cleric, U.S. administrator L. Paul Bremer said the coalition would help strengthen border security, saying it was “increasingly apparent” that “a large part of terrorism” comes from outside Iraq.

Grand Ayatollah Ali al-Husseini al-Sistani and other Shiite leaders accused the coalition of failing to provide adequate security for the worshippers and of not doing enough to prevent extremists from crossing Iraq’s porous borders.

“There are 8,000 border police on duty today and more are on the way,” Bremer said. “We are adding hundreds of vehicles and doubling border police staffing in selected areas. The United States has committed $60 million to support border security.”

Mourners carry the coffin of one of the victims of Tuesday's suicide attacks for final rites at the Imam Hussein Mosque in Karbala, Iraq. The series of explosions at the Shiite shrines in Karbala as well as in Baghdad killed at least 150 people. Dozens of funerals took place Wednesday.