A soldier with the 4th Infantry Division was killed and another wounded in an explosion of a makeshift bomb near Tikrit, the U.S. Central Command said Monday. Meanwhile, 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. Furthermore, the U.S. Army retrieved the wreckage of a downed transport helicopter, searching for clues about who knocked it from the sky the day before. On Tuesday, a military spokesman said that a mortar round or a rocket had struck the so-called "Green Zone" - the heavily defended area in central Baghdad that houses the U.S.-led administration. He said the blast, one several heard late Tuesday 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. 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. "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 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 US dollars apiece for each portable missile turned in but has refused to say how many have been surrendered. Also, the explosion in Karbala, 65 miles south of Baghdad, apparently was caused by a bomb planted in a parked car on a busy street less than 100 yards from the gold-domed Imam Hussein shrine, said Mohammed Abu Jaffar al-Assadi, a Shiite cleric. Other witnesses said it might have been concealed in a bag left outside a hotel. In addition to at least one dead, it was believed 12 people were wounded, al-Assadi said. It was not immediately possible to get confirmation. As a result of Sunday's shootdown, the U.S. command may have to re-evaluate the routes and flying tactics of its transport helicopters and planes over Iraq. The SA-7 Strela portable missiles known to have been in Iraqi hands, weapons that home in on the engine heat of an aircraft, can be fired to an altitude of 14,000 feet, easily covering the usual cruising altitude of a heavily laden Chinook. Another shoulder-fired missile in the old Iraqi army's inventory, the advanced SA-18 Igla, is equipped with special filters to defeat flares and other countermeasures deployed by U.S. aircraft. The apparent successful use of such a weapon in Sunday's attack is a sign of the increasing sophistication of Iraq's elusive anti-U.S. resistance, whose attacks have intensified in recent weeks. At the site Monday, a giant crane lifted pieces of wreckage onto a truck, as soldiers sealed off the immediate area. Villager Jamal Abed, 22, said U.S. troops came to his house Monday morning and told him, through an interpreter, that "if American forces were subjected to fire, they will open fire on every house in the area." In other developments: · A neighborhood council chairman in west Baghdad, Mustafa Zaidan al-Khaleefa, 47, was fatally shot from a passing car late Sunday. Numerous Iraqi local and national officials cooperating with the occupation have been targeted for assassination. · In the southern city of Basra, some 1,500 members of a new Iraqi security guard force protested outside the mayor's office, seeking a higher bonus for Ramadan. **PHOTO CAPTION*** U.S. Army 4th Infantry Division soldiers tie up Spc. Jeffery Barnaby, center, with tape as they celebrate his 26th birthday at the military base in Tikrit, 193 km north of Baghdad, Iraq, Saturday Nov. 1, 2003. (AP Photo/Ivan Sekretarev)