BAGHDAD, Iraq - The U.S. military deployed armored vehicles in Mosul and infantrymen swept through the northern city today, a day after an insurgent strike on a nearby base killed 22 people and injured 72 in one of the deadliest attacks on American troops since the war began.

The military was investigating whether a bomb was planted at the mess tent in Forward Operating Base Marez, where the blast sprayed shrapnel as U.S. soldiers sat down to lunch Tuesday. Initial reports said a 122 mm rocket ripped through the tent's ceiling.

Lt. Gen. Thomas Metz, commander of the multinational force in Iraq, told CNN that a planted bomb was "a possibility."

A radical Sunni Muslim group, the Ansar al-Sunnah Army, which claimed responsibility for the attack, said it was a "martyrdom operation" - a reference to a suicide bomber.

Metz told CNN that experts had flown from Baghdad to Mosul to "do a very detailed explosive forensics investigation and they will be able to tell us the type of weapon (and) the size of weapon" that was used.

He also said the military was looking at better ways of protecting dining areas, gyms and other places where troops gather in large numbers on bases. Mortar attacks on U.S. bases, particularly on the huge white tents that serve as dining halls, have been frequent in Iraq for more than a year. Bill Nemitz, a columnist with the Portland (Maine) Press Herald who was embedded with the troops in Mosul, told CNN that he heard "a lot of discussion" among troops about the vulnerability of the tent.

Mosul's streets were deserted today as hundreds of troops spread out across several neighborhoods backed up by Bradley fighting vehicles and armored Humvees. An Associated Press reporter in Mosul saw helicopter gunships clattering overhead and jets flying high above the city, located 225 miles north of Baghdad.

Metz told CNN that previous attacks on Marez were "rather random."

"The enemy cannot stay in one place long to attack us, therefore his accuracy is pretty poor," Metz said.

U.S. troops blocked Mosul's five bridges over the Tigris River that link the western and eastern sectors of the city. The AP reporter saw U.S. soldiers conducting sweeps through the eastern neighborhoods of Muthanna, Wahda and Hadabaa.In a sign of the simmering tensions, most schools in the city were closed and few cars and people could be seen on the streets, although a formal curfew was not declared. Even traffic policemen were not at major intersections as usual.The dead from the blast included 18 Americans - 14 service members and four U.S. civilian contractors - and four Iraqis, the U.S. military command in Baghdad said today. Of the 72 wounded, 51 are U.S. military personnel and the remainder are American civilians, Iraqi troops, and other foreigners.Defense contractor Halliburton Co. said four of its employees and three subcontractors were killed, but it did not provide names or nationalities, and the U.S. military did not provide a further breakdown of the identities of the dead.At the military hospital near Mosul airfield, doctors and orderlies treated dozens of soldiers for burns, shrapnel wounds and damage to their eyes."This is the worst we have seen in the 11 months since we have been here," said Master Sgt. David Scott, chief ward master for the hospital.It was the latest in a week of deadly strikes across Iraq that highlighted the growing power of the insurgents in the run-up to the Jan. 30 national elections."We are not going to be intimidated. We will help the Iraqi people and their security forces not to be intimidated and we are pushing on toward the elections in January," Metz said.President Bush said the explosion should not derail the elections and that he hoped relatives of those killed know that their loved ones died in "a vital mission for peace.""I'm confident democracy will prevail in Iraq," he said.There was little apparent sympathy for the dead Americans on Mosul streets today."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 Tuesday's blast, is believed to be a fundamentalist group that wants to turn Iraq into an Islamic state like Afghanistan's former Taliban regime. The Sunni group claimed responsibility for the execution of 12 Nepalese hostages and other recent attacks in Mosul.Immediately after the explosion, soldiers turned their lunch tables upside down, placed the wounded on them and gently carried them into the parking lot, said Jeremy Redmon, a reporter for the Richmond (Va.) Times-Dispatch embedded with the troops in Mosul.Redmon said the dead included two soldiers from the Richmond-based 276th Engineer Battalion, which had just sat down to eat. The force of the blast knocked soldiers off their feet and out of their seats as a fireball enveloped the top of the tent and shrapnel sprayed into the area, Redmon said.Scores of troops crammed into concrete bomb shelters, while others wandered around in a daze and collapsed, he said. "I can't hear! I can't hear!" one female soldier cried as a friend hugged her.Sgt. Kyle Wright said he was about to take a bite of chocolate cake when the blast knocked him out of his chair. Two other Virginia National Guardsmen picked him up and rushed him out of the tent."I kind of went into the air," Wright said as he lay in a hospital near Mosul airfield, recovering from wounds to his leg and back. "When I came to, I looked up and I saw open sky."In other developments today:Poland's Prime Minister Marek Belka and Defense Minister Jerzy Szmajdzinski toured Camp Echo in Diwaniyah, the new headquarters for the Polish-led international security force in central Iraq, for a Christmas visit to some 2,400 Polish troops stationed in Iraq.Four Iraqi civilians from one family were killed and three others were injured when U.S. soldiers opened fire on their car in the Abu Ghraib area just west of Baghdad, said Akram Al-Zaobaie, a doctor in the local hospital.The seven were traveling in a taxi when a roadside bomb hit an American military convoy, prompting fire from the U.S. soldiers, he said.Iraqi security forces stormed a house in the Shiite Muslim holy city of Najaf in a shootout that killed one guerrilla and a policeman. The raid came after 54 people were killed and 142 injured in a car bomb explosion Sunday in Najaf's city center.Meanwhile, the International Committee of the Red Cross said today its experts visited the devastated town of Fallujah to assess the humanitarian situation there ahead of a planned return of some 250,000 civilians that is to start this week.Water purification plants in Fallujah remain dysfunctional after the U.S.-led offensive that left the city in ruins, and returning displaced families will have to depend on mobile tanks, said Ahmed Rawi, an ICRC spokesman.And unknown gunmen shot dead an Iraqi police officer in Baqouba, northeast of Baghdad, Sgt. Hussein Hassan said. The assailants stole the victim's pistol before fleeing.