Helicopters and warplanes patrolled overhead as troops searched for insurgents, reports CBS News Correspondent Cami McCormick.
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 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 bomb was apparently packed with pellets the size of BBs that ripped across the tent when it exploded, Brig. Gen. Carter F. Ham, commander of Task Force Olympia — the main U.S. force in northern Iraq — told newspaper columnist Bill Nemitz.
The military was also 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. 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.
In other recent developments:
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."
The dead included 18 Americans — 13 service members and five U.S. civilian contractors — and three Iraqi National Guard members as well as one unidentified non-U.S. citizen, the U.S. military command in Baghdad said Wednesday.
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. The Sunni group claimed responsibility for the execution of 12 Nepalese hostages and other recent attacks in Mosul.
Relatives of troops waited anxiously for word of their loved ones. A news photo of Sgt. Evan Byler, his blood-stained hand clutching a cigarette, was a special Christmas gift for his fiancée Michele Gibson of Virginia.
"I was pretty much hysterical all afternoon until I at least saw the picture and saw that he was okay," she tells CBS News Correspondent Thalia Assuras. "I feel very bad for the families that lost whoever they lost, but I feel thankful that he's okay."
The casualties come from units across the country. In Fort Lewis, Wash., Leslie Swope waits for word of her husband. "It's very hard, but they're over there for a reason," she says. "He wouldn't have it any other way."