BAGHDAD, Iraq – The American military said yesterday that a suicide bomber likely was responsible for the explosion at an American base near Mosul on Tuesday that sprayed a crowded mess tent with small pellets and killed 22 people – nearly all of them Americans.

The announcement raised questions about how the attacker infiltrated the base, which is surrounded by blast walls and barbed wire and guarded by American troops. However, as in many other American military facilities, Iraqis do a variety of jobs at the base, including cleaning, cooking, construction, and office duties.

The apparent sophistication of Tuesday’s operation – the deadliest single attack on American troops since the war began – indicated the attacker probably had inside knowledge of the base’s layout and the soldiers’ schedules. The blast came at lunchtime.

“We have had a suicide bomber apparently strap something to his body…and go into a dining hall,” General Richard Myers, the chairman of the Joint Chiefs of Staff, told reporters at the Pentagon. “We know how difficult this is to prevent people bent on suicide and stopping them.”

Lieutenant General Thomas Metz, ground forces commander in Iraq, ordered an investigation. Troops found “no physical evidence of a rocket, mortar, or other type of indirect fire weapon,” according to a statement issued early today by military authorities in Baghdad.

There was little apparent sympathy for the dead Americans on Mosul’s deserted streets, where hundreds of American troops, backed up by armored vehicles and helicopters, blocked bridges and cordoned off Sunni Muslim areas of Iraq’s third largest city.

“I wish that 2,000 U.S. soldiers were killed,” declared Jamal Mahmoud, a trade union official.

Initial reports said a rocket had ripped into the tent. Later, however, a radical Sunni Muslim group, the Ansar al-Sunnah Army, claimed responsibility, saying it was a “martyrdom operation” – generally a reference to a suicide bomber.

Military officials in Iraq said yesterday that shrapnel from the explosion included small ball bearings, which are often used in suicide bombings, but are not usually part of shrapnel from rockets or mortars.

The attack sparked renewed concerns about the ability of American troops and their Iraqi allies to secure elections January 30. The military said they had expected an increase in violence as insurgents attempt to derail the vote for an assembly that will draft Iraq’s new constitution.

“Insurgents, who have everything to lose, are desperate to create the perception that elections are not possible,” said General George Casey, the commander of multinational forces in Iraq. “We will not allow terrorist violence to stop progress toward elections.”

Mortar attacks on American bases, particularly on the huge white tents that serve as dining halls, have been frequent in Iraq for more than a year. Just last month, a mortar attack on a Mosul base killed two troops with Task Force Olympia, the main force responsible for security in northern Iraq.

Tuesday’s blast wrecked the mess tent at Forward Operating Base Marez, a military camp for American and Iraqi government forces just south of Mosul.

The 22 dead included 14 American service members, four American civilians, three Iraqi National Guard members, and one “unidentified non-U.S. person,” the American military command in Baghdad said last night.

General Myers said authorities don’t know whether the unidentified person was the likely bomber. Of the 69 wounded, 44 are members of the American military, seven are American contractors, five are civilian workers for the Defense Department, two are Iraqi civilians, 10 are contractors of other nationalities, and one is of unknown nationality and occupation, the statement said.

About 50 people – most of them injured soldiers from Mosul – arrived in Germany yesterday aboard an Air Force C-141 transport plane. As a light snow fell, some wounded were carried away on stretchers. Halliburton Co. lost four American employees in the attack, the Houston-based contractor said. Sixteen other Halliburton workers, including 12 subcontractors, were injured seriously.

In the immediate aftermath of Saddam Hussein’s ouster in April 2003, American commanders cited Mosul – with a population of 1.2 million some 220 miles north of Baghdad – as a success story. But armed opposition has mounted, especially since last month’s successful American-led operation to retake the insurgent-held town of Fallujah.

Many insurgents apparently moved to Mosul, where guerrillas launched a coordinated surprise attack in November against police stations. The municipal police force, estimated at over 6,000 officers, disintegrated; despite the success of American troops a few days later in re-establishing control, only part of the police force has returned to work.