"At this point, it looks like it was an improvised explosive device worn by an attacker," Gen. Richard Myers, the chairman of the Joint Chiefs of Staff, told reporters at a Pentagon news conference.
Tuesday's attack on a base in the northern city of Mosul killed 22 people and injured 69, the U.S. military command in Baghdad said. Of the wounded, 44 were U.S. military personnel and the rest American civilians, Iraqi troops and other foreigners.
The dining hall explosion happened while hundreds of troops were eating lunch Tuesday at Forward Operating Base Marez near Mosul. Military officials initially cast doubt on the idea that a suicide bomber had been the cause, saying a 122mm rocket was the likely cause.
Myers declined to say what evidence showed the attack was a suicide bomb rather than a rocket attack.
Military officials in Iraq on Wednesday said shrapnel from the explosion included small ball bearings, which are often used in suicide bombings but are not usually part of the shrapnel given off by rockets or mortars.
Meanwhile, a group of 42 U.S. soldiers and civilians wounded in the deadly insurgent attack were flown to the Landstuhl Ramstein Regional Medical Center for treatment Wednesday, many of them brought out in stretchers as a light snow fell.
In other recent developments:
With the Christmas holidays near, extra staff were told to be on standby to help care for the 35 U.S. troops and seven civilians, who arrived in Germany from Balad, north of Baghdad.
Eight patients were in intensive care, hospital spokeswoman Marie Shaw said. She had no details on their condition, but said the injured generally have "chest wounds and shrapnel wounds and broken bones."
In Mosul, U.S. troops backed by armored vehicles swept through the virtually empty streets amid an undeclared curfew in Iraq's third largest city Wednesday, a day after one of the deadliest attacks on American troops.
A radical Sunni group, the Ansar al-Sunnah Army, said it carried out the attack and claimed it was a "martyrdom operation" — a reference to a suicide bomber.
The military was looking at better ways of protecting places where U.S. troops regularly gather on their bases, such as dining areas and gyms — areas that are frequently targeted by mortars, though usually with little accuracy. Newspaper columnist Bill Nemitz, embedded with the troops at Marez for the Portland (Maine) Press Herald, told CNN that he heard "a lot of discussion" among troops about the vulnerability of the tent.
An Associated Press reporter saw almost no cars or people on the streets of Mosul Wednesday and most schools in the city were closed, although a formal curfew was not declared. Even traffic policemen were not at major intersections as usual.
U.S. forces blocked Mosul's five bridges over the Tigris River that link the western and eastern sectors of the city, while hundreds of troops spread out across several neighborhoods, conducting sweeps in eastern districts backed by Bradley fighting vehicles and armored Humvees.
The blast came as the military had nearly finished building a reinforced, bunker-like dining area at the camp to increase protection against mortar and rocket attacks, Metz said. The new facility was due to be completed in February, he said.
"We recognized the threat," Metz said Wednesday on CBS News' The Early Show
Metz told CNN that previous rocket and mortar attacks on Marez were "rather random."
Of the 22 people killed in the mess tent Tuesday, 13 were U.S. military, five civilian contractors, three Iraqi security force members and one other person.
Of the 69 wounded, 44 were U.S. military personnel and the remainder American civilians, Iraqi troops, and other foreigners.
Defense contractor Halliburton Co. said four of its employees were killed.
There was little apparent sympathy for the dead Americans on Mosul streets Wednesday.
"In fact, what has happened in Mosul yesterday is something expected," said Sattar Jabbar. "When occupiers come to any country (they) find resistance. And this is within Iraqi resistance."
"I prefer that American troops leave the country and go out of cities so that Iraq will be safer and we run its affairs," Jamal Mahmoud, a trade union official. "I wish that 2,000 U.S. soldiers were killed, not 20."
The Ansar al-Sunnah Army, which claimed the responsibility for Sunday's blast, is believed to be a fundamentalist group that wants to turn Iraq into an Islamic state like Afghanistan's former Taliban regime.