Baghdad, Iraq ? American troops hunted for anti-aircraft missiles along Iraq’s trucking routes, digging through heaps of manure, mounds of hay or piles of pomegranates Monday. The U.S. Army retrieved the wreckage of a downed transport helicopter and searched for clues about who knocked it from the sky.

Attacks continued Monday — a soldier with the 4th Infantry Division was killed and another wounded in an explosion of an improvised bomb near Tikrit, the U.S. Central Command said, and witnesses reported that a blast near a Shiite Muslim shrine in the southern city of Karbala killed at least one person.

Today, a military spokesman said that a mortar round or a rocket had struck the “Green Zone” — the heavily defended area in central Baghdad that houses the U.S.-led administration. He said the blast, one of several heard in the capital, caused no damage or casualties.

One clue in Sunday’s helicopter shootdown may lie in Ramadi, west of the crash site, where an anti-U.S. leaflet warned, just two days before the shootdown, that Iraq’s insurgents would strike the Americans with “modern and advanced methods.”

The downing of the CH-47 Chinook, one of two carrying dozens of soldiers on their way to Baghdad airport and home leave, killed 16 Americans and wounded 20 others. It was the heaviest U.S. death toll in any single action since the invasion of Iraq last March 20.

Sixteen of the injured were flown by U.S. Air Force C-17 transport Monday to Ramstein Air Base in Germany and treated at the U.S. military’s Landstuhl Regional Medical Center. Nine were admitted to the intensive care unit, including five in serious condition, said hospital spokeswoman Marie Shaw.

“They are being evaluated and surgeries are planned throughout the day,” she said.

Villagers who saw the helicopter downing south of Fallujah, 35 miles west of Baghdad, said it was struck from behind by one or two missiles apparently fired from a date palm grove in the area, deep in the Sunni Muslim heartland that has produced the most violent opposition to the U.S. occupation of Iraq.

CBS Evening News quoted one wounded survivor at a U.S. military hospital in Baghdad shortly after the crash. Cpl. David Tennant said the missile hit the back of the Chinook, and the helicopter caught fire before it went down.

A U.S. Army soldier from the 720th Military Police Battalion blocks Highway 1, Iraq's main north-south thoroughfare, outside of Tikrit, 120 miles north of Baghdad. The battalion, assisted by Iraqi police, was searching for contraband weapons that could be used by guerrillas, a routine operation that has taken on added urgency since insurgents Sunday shot down a Chinook helicopter with a missile, killing 16 soldiers and wounding 20 others.

“Everybody was just laid out everywhere, and they were trying to search for most of the people that were left within the rubble. There was a lot of people screaming,” Tennant told CBS. “I just remember waking up in the middle of the rubble, trying to escape, trying to get out of the burning metal.”

Hundreds of portable, shoulder-fired missiles are unaccounted for in Iraq, potential threats to a U.S. occupation army that relies heavily on the slow, low-flying CH-47 Chinook craft for troop transport. The U.S. command has offered Iraqis $500 apiece for each portable missile turned in but has refused to say how many have been surrendered.