Iraqi officials said the death toll from Tuesday's coordinated series of blasts against religious pilgrims jumped dramatically as a U.S. official said 15 people were being questioned in connection with the deadly explosions.

"We think these people were involved, and that's why they're being interrogated," said U.S. Brig. Gen. Mark Kimmitt (search). The 15 people were detained in Karbala after being pointed out by witnesses. Some had been seen with wooden carts used to bring in explosives.

If implicated in the violence, they would be charged with taking part in the deadliest day in Iraq since the end of major combat operations in the U.S.-led war against Saddam Hussein.

Initially, the death toll was put at around 140 people, but the number fluctuated up and down all day Tuesday. U.S. officials now put the death toll at 117.

But Iraq Governing Council President Mohammed Bahr al-Ulloum (search) said Wednesday that 271 people were killed and 393 wounded in the near-simultaneous bombings at Baghdad's Kadhimiya shrine and holy sites in Karbala. It was impossible to immediately reconcile the discrepancy between the U.S. and Iraqi figures.

On Wednesday, Shiite Muslim (search) mourners chanted slogans against the United States, venting their anger at Iraq's instability after the homicide bombings. The country is now in three days of mourning.

In other news from Iraq: 

— The draft constitution drawn up this week by Iraq's leading politicians and the U.S.-led administration will be signed Friday as the national period of mourning comes to an end, al-Ulloum said.

— Two large explosions were heard Wednesday in central Baghdad and appeared to have come from an area to the west of the capital. There were no immediate details on the cause of the blast.

Tuesday's bombings — using homicide attackers and bombs brought in on wooden carts — struck pilgrims from Iraq, Iran and other Shiite communities who had gathered to mark Ashoura, the holiest day of the Shiite calendar.

Iran said at least 22 of its citizens were among the dead. Deputy Interior Minister Ali Asghar Ahmadi urged Iranian pilgrims Wednesday to put off visiting Iraq in the wake of the blasts.

Among the 15 detained were five Farsi speakers, Kimmitt said, a suggestion that they were Iranians. Kimmitt said the 10 others appeared to be Iraqis. An estimated 100,000 Iranians were believed to have come to Iraq for Ashoura.

Three homicide bombers carried out the blasts at Kadhimiya, one inside the shrine's courtyard and two outside, Kimmitt said. Early reports that a would-be fourth homicide bomber was caught proved incorrect, he said. The Karbala attack involved one homicide bomber and cart-borne bombs set up on nearby roads. Mortars may also have been fired on the city outskirts, he said.

As authorities slowly identified the dead, relatives picked up their slain loved ones from Karbala's Al-Hussein hospital Wednesday. Others wept as they scanned handwritten lists of names posted on the hospital walls. Iranian pilgrims, speaking in Farsi, struggled to communicate with the Iraqi hospital officials.

Several thousand joined a funeral procession in the afternoon, taking three bodies to the tombs of Islamic saints Imam Hussein and Imam Abbas for blessings before heading to bury then at the cemetery in this city 50 miles south of Baghdad.

"No, no, Americans! No, no Israel! No, no, terrorists!" they chanted, carrying red, black and green flags, symbols of martyrdom traditionally used for Ashoura ceremonies.

U.S. and Iraqi officials pointed to an Al Qaeda (search)-linked Jordanian militant, Abu Musab al-Zarqawi, as a "prime suspect" in the attacks, saying he aims to spark a Shiite-Sunni civil war in Iraq.

The dramatic attacks forced the delay of a key milestone in the U.S. handover schedule — the planned Thursday signing of an interim constitution.

Iraqi leaders have worried about Shiite revenge attacks against Sunnis and pleaded with the public to maintain unity.

Many Iraqis, including Shiites, have also blamed foreigners — throwing suspicion on Al Qaeda. But the focus of Shiite anger has been directed more at the U.S.-led occupation. Some, including the top Shiite cleric, accused U.S. officials of not doing enough to protect the 10-day Ashoura ceremonies; others simply vented resentment over the country's continuing insecurity.

A letter purported to come from Al Qaeda denied responsibility for the bombings, blaming American troops instead — but it also called Shiites infidels.

"The American troops have carried out a massacre to kill innocent Shiites in Karbala, their (Shiites') infidel city, and in Baghdad," the letter, received on Wednesday via e-mail by the London-based Al-Quds Al-Arabi newspaper and shared with The Associated Press in Cairo.

Also Wednesday, three rockets hit a telephone exchange building in Baghdad, knocking out international phone service for much of the country only days after the system was put back in service. One Iraqi worker was killed and another injured, Iraqi officials said.

Restoring telephones knocked out during the U.S. invasion last year has been a priority in U.S. efforts to bring back a sense of normalcy for Iraqis.

It appeared that other attacks had been planned for Tuesday.

In Kirkuk, police found and defused a 22-pound bomb alongside a road where Shiites had planned to march Tuesday, said Anwar Amin, the Iraqi Civil Defense Corps chief in Kirkuk.

In Najaf, police arrested two people carrying explosives near the Imam Ali shrine, police Col. Saeed al-Joubri said Wednesday.

Police in Basra said they found a car packed with 550 pounds of explosives and two women who were apparently planning to set explosives in Shiite mosques. But Kimmitt denied those reports, calling them only rumors. "None of them have borne out to be correct," he told reporters.

The Associated Press contributed to this report.