Baghdad ? U.S. troops flooded a Baghdad neighborhood and fired a missile at an alleged training base in northern Iraq, part of a new offensive against guerrillas.

The military moves came as the Army tried to determine why two of its Black Hawk helicopters crashed in the northern city of Mosul on Saturday, killing 17 soldiers in the worst single loss of American life since the war began.

Sunday, the military fired a satellite-guided missile with a 500-pound warhead from Taji, north of Baghdad, and hit a suspected training base west of Kirkuk, about 130 miles away, said Lt. Col. William MacDonald, spokesman of the 4th Infantry Division.

MacDonald said it was the first time such missiles have been used since the end of major combat on May 1.

In Baghdad, U.S. troops backed by armored vehicles and helicopters moved into the Sunni Muslim neighborhood of Azamiyah, sealing off a 20-block area and searching 450 houses over seven hours. They netted 30 Kalashnikovs, about a dozen shotguns and 10 pistols, according to U.S. officers.

Soldiers detained 21 people for illegal weapons possession, although all were expected to be released today.

The raids in Azamiyah, an upper-class neighborhood, angered many residents.

“Of course everybody has weapons. We have all been robbed. We were afraid of the Iraqis and now we’re afraid of the Americans,” said Samir al-Hadith, an engineer from Saudi Arabia.

In northern Iraq, the U.S. military was investigating whether ground fire from guerrillas caused Saturday’s collision of the two U.S. helicopters.

Soldiers remove the wreckage of an U.S. Army Black Hawk helicopter from a residential area in Mosul, northern Iraq.

“There are reports that there may have been ground fire, and one of them may have been trying to avoid that. We just don’t know at this point,” said L. Paul Bremer, the top U.S. administrator in Iraq.

All the victims were from the 101st Airborne Division, which is based in Fort Campbell, Ky., a military spokesman said. Division spokesman Maj. Trey Cate said one helicopter carried a quick response team that was on its way to investigate a shooting incident in which a U.S. soldier was injured. The other helicopter was on a transport mission.

An Iraqi policeman in Mosul said at least one of the Black Hawks was hit by ground fire.

“They hit it with a missile,” said policeman Saddam Abdel Sattar. “I was in the army. I know these things.”

Another witness said he heard gunfire on the ground before the crash.

“The Black Hawks were in the air and there was shooting. It was dark and one slammed into the other,” said an Iraqi Civil Defense Corps soldier who identified himself only as Mahmoud.

Before the Saturday crash, the U.S. military’s deadliest single incident since the Iraq war began March 20 was the downing of a Chinook helicopter near Fallujah on Nov. 2 that killed 16 soldiers. A Black Hawk was also shot down on Nov. 7 in Tikrit, killing all six soldiers on board.

In other violence, a roadside bomb detonated under an Army convoy in Mosul, wounding five U.S. soldiers. The military also said four Iraqi insurgents were killed late Saturday in two separate clashes with U.S. troops in Diyala province.