A bomber blew himself up in a crowd of Iraqis outside an army recruitment center Tuesday, killing 21 other people and injuring 27 more, the U.S. military said. It was the deadliest attack in the Iraqi capital since last week's election.

There were conflicting reports about the attack, which occurred at an Iraqi National Guard (search) headquarters at the Muthana airfield. Iraqi officials blamed the explosion on mortar fire and officials at Baghdad's Yarmouk Hospital said they had received 16 bodies from the scene, all of them army recruits.

But witnesses reported only one explosion, and the U.S. military said the blast was caused by a bomber.

The Al Qaeda in Iraq (search) terror group, led by Jordanian militant Abu Musab al-Zarqawi (search), claimed responsibility for the attack in an Internet statement.

"This is the beginning of the escalation we promised you," said the statement posted on an Islamic Web site. The claim's authenticity couldn't be verified.

Elsewhere, three police officers were killed in clashes in Baghdad's western Ghazaliya neighborhood, scene of numerous clashes and assassinations over the past six months.

Also Tuesday, assailants sprayed a politician's car with gunfire, killing two of the man's sons, an Interior Ministry official said. The politician, Mithal al-Alusi, who heads the Nation party, escaped unhurt.

He gained notoriety last year after he was expelled from the Iraqi National Congress party for attending a terrorism conference in Israel. Al-Alusi is one of the candidates who ran in Iraq's landmark Jan. 30 elections.

On Monday, gunmen killed an Iraqi chef employed by U.S. forces at Baghdad International Airport, hospital officials said Tuesday. In Mosul, two Kurdish politicians were also gunned down Monday, said an official from the Kurdistan Democratic Party (search).

The violence is picking up again in the Iraqi capital following the elections, when a massive security crackdown prevented insurgents from launching major attacks. Iraqis chose a 275-member National Assembly and provincial councils, as well as a regional parliament in the Kurdish-controlled north.

Final results of the election are expected this week. The latest partial returns released Tuesday showed a Kurdish ticket had moved into second place behind a coalition of Shiite religious parties, relegating a faction led by U.S.-backed interim Prime Minister Ayad Allawi (search) to third place.

First election returns from the Sunni heartland — including Saddam Hussein's hometown of Tikrit — confirmed Monday that many Sunnis stayed away from the ballot box, leaving the field to Shiite and Kurdish candidates. A Shiite-dominated ticket backed by the Shiite clergy leads among the 111 candidate lists, with a final tally of last week's National Assembly election expected by week's end.

Allawi, who favors strong ties with the United States, had hoped to emerge as a compromise choice for prime minister, but those on the Shiite cleric-backed ticket say they want one of their own for the top job.

Kurds, estimated at 15 percent to 20 percent of the population, gave most of their votes to a joint ticket made up of the two major Kurdish parties, which was in second with about 24 percent of the votes reported as of Monday. One of the Kurdish leaders, Jalal Talabani, has announced his candidacy for the presidency.

Allawi's ticket trailed with about 13 percent of the vote, with the Shiite ticket leading with about half the votes. Shiites comprise about 60 percent of Iraq's 26 million people.

Also Tuesday, a militant group claimed in an Internet statement that it has executed a female Italian journalist abducted in Baghdad for spying on "holy fighters."

There was no way to verify the authenticity of the statement, which offered no proof that Giuliana Sgrena (search), a 56-year-old reporter, had been held by the group. She was kidnapped by gunmen on Friday in Baghdad.

Tuesday's attacks were the latest sign that insurgents are stepping up their assault on Iraq's security forces, which the United States hopes can assume a greater role once a newly elected government takes office.

A day earlier, a bomber also wandered into a crowd of Iraqi policemen near a hospital in the northern city of Mosul, killing 12 officers.

The attacks by bombers on foot point to a shift in rebel tactics, as concrete blast walls and roadblocks have made it harder for guerrillas to strike at Iraqi security forces with car bombs.

The bombings, shootings and kidnappings have shattered a brief downturn in violence after the elections, the first nationwide balloting since the fall of Saddam in April 2003.

Election officials acknowledged thousands of people in the Sunni-dominated Mosul area who wanted to vote during the balloting were unable to because of security. Fewer than a third of the planned 330 polling centers in Mosul and the surrounding province managed to open on election day, officials said.