BAGHDAD, Iraq — U.S. troops backed by armored vehicles swept through virtually empty streets of Mosul amid an undeclared curfew in Iraq's third largest city Wednesday, a day after an insurgent strike on a nearby base killed 22 people and wounded 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 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 explosive was apparently packed with pellets the size of BBs that ripped acorss the tent when it exploded, Brig. Gen. Carter F. Ham, commander of Task Force Olympia — the main U.S. force in nothern Iraq — told Nemitz.

Mortars and rockets produce shrapnel but are not packed with pellets, which are often found in roadside bombs or explosives worn by suicide bombers.

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. Bill Nemitz, a columnist with the Portland (Maine) Press Herald who was embedded with the troops at Marez, told CNN that he heard "a lot of discussion" among troops about the vulnerability of the tent.

About 50 people — most of them injured soldiers from Mosul — arrived on an Air Force C-141 transport plane at Ramstein Air Base in Germany for treatment at nearby Landstuhl Regional Medical Center, said Maj. Mike Young, a base spokesman.

The hospital was expected at least eight who were in critical condition, Landstuhl spokeswoman Marie Shaw said. With a light snow falling, some wounded were carried out on stretchers, while about a dozen were expected to be well enough to walk off the plane.

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.