Also, near the flashpoint city of Fallujah, three civilians were killed and two wounded when their convoy came under fire. An American engineer and an Iraqi security guard said U.S. troops shot at their vehicles, but the military denied that.
Amid the ongoing violence, U.S military officials prepared for the holy fasting month of Ramadan, which begins in Iraq on Monday. For weeks, chaplains have been training troops to be sensitive to Muslim religious traditions.
On Saturday, U.S. forces reopened a major bridge over the Tigris River to ease transportation in the capital, Baghdad. An American military brass band with a tuba played as the bridge was inaugurated.
And the overnight curfew in effect in Baghdad since coalition troops took the capital in April was due to be lifted early Sunday morning.
In other developments:
The Black Hawk came down at about 4 p.m. local time in a field near Tikrit. Both it and Fallujah lie inside the "Sunni Triangle," which sees multiple attacks every day against U.S. forces. The region, in central Iraq north of Baghdad, is where the ousted dictator drew his strongest support; his loyalists are now believed to be leading resistance to the U.S.-led occupation.
A U.S. military spokesman in Baghdad, who spoke on condition of anonymity, said the helicopter landed for undetermined reasons. He said the Black Hawk — aircraft that usually carry a crew of three — was supporting a combat patrol.
A preliminary report by U.S. soldiers, however, said the copter was apparently downed by ground fire, possibly by an RPG. U.S. officials have been warning that thousands of shoulder-fired anti-aircraft missiles remain unaccounted for after the fall of Saddam's regime and pose a threat to U.S. military aircraft.
Witnesses said they heard a loud explosion as two helicopters flew low overhead. Suddenly, one of the aircraft began swaying from side to side and came down about a mile away.
Only one U.S. helicopter has been confirmed shot down by hostile fire since Mr. Bush declared an end to major combat in Iraq on May 1. A U.S. Army Apache attack helicopter was shot down June 12 by hostile fire in western Iraq. The craft's two crewmembers were rescued unhurt.
In the incident near Fallujah, three SUVs of the European Landmine Solutions, a British-based private contractor, were hit by gunfire, according to an American engineer with the firm, David Rasmussen, who was hospitalized with wounds.
Asked where the shots came from, Rasmussen replied: "from the USA."
The Iraqi security guard traveling with the convoy, Laith Yousef, gave the same account.
"We were the target of an attack by the Americans," Yousef said. "They shot at our car. The translator burned to death in the car. A man with us was killed. He was going to get married next week."
A U.S. command spokesman in Baghdad denied troops fired at the convoy, saying that, instead, coalition forces went to the secure the area after the attack and evacuated the wounded.
"According to my information, it's impossible they were attacked by U.S. forces," he said. "They weren't close enough to either cause or prevent this attack."
Lt. Col. Charles Hardy, a spokesman for the 82nd Airborne Division which is responsible for the area, said the civilian convoy turned around after a road bomb exploded ahead of it, and then was hit by another exploding improvised bomb and by rocket-propelled grenade and small-arms fire.
"U.S. forces arrived after the attack and treated the wounded," he said. "This was not initiated by our forces."
Deputy Secretary of Defense Paul Wolfowitz, on a three-day tour of Iraq, was in Tikrit earlier Saturday visiting a U.S. garrison. He left the city hours before the helicopter came down and was in the northern city of Kirkuk, U.S. officials said.
Before leaving Tikrit, Wolfowitz, who arrived Friday, told commanders he was hopeful American troops would get more money to train Iraqis to assume a greater role in security as they fight resistance forces.
"These young Iraqis are stepping forward to fight for their country along with us," Wolfowitz said. "It is a wonderful success story that speaks volumes."
Iraqis who work with coalition forces have been frequently targeted by insurgents opposed to the U.S. occupation. On Saturday, officials said the coalition-backed police chief of the southern Iraqi city of Amarah was shot to death.
Brig. Hamid Hadi Hassan al-Abe was leaving the al-Hussein mosque after Friday prayers when he was gunned down by assailants firing from several locations, police Maj. Kathim Mohsen Hamadi said. The attackers escaped.
Amarah is populated primarily by Shiite Muslims, who have been generally more accepting of the occupation because of their suffering under former Sunni-dominated regime. However, Hamadi said Iraqis who work closely with the coalition are often considered traitors.
In the Bulgarian capital Sofia, the Foreign Ministry said it had asked its diplomatic staff to leave Baghdad and move to Amman, citing "the current priorities of our diplomatic mission." The Bulgarian newspaper Trud said the decision was taken because of threats of terrorist attacks.
The newspaper said the 50 Bulgarian citizens in Baghdad had been advised to leave, too. Bulgaria has about 480 troops in Iraq as part of the U.S.-led mission.
Meanwhile, in preparation for Ramadan, coalition officials moved to ease conditions for Baghdad's 5 million people by abolishing the nighttime curfew and reopening the Tigris bridge.
The Islamic month of fasting begins with the sighting of a new moon and lasts for four weeks. Muslims must abstain from food, drink and sex during the day, but evenings are marked by lavish meals and social gatherings lasting far into the night.
"The curfew can be lifted due to the reduction in the crime rate in the city and the overall improvement in the security situation," the U.S.-appointed Baghdad city council statement said. "Despite some highly publicized attacks by terrorists and supporters of the former regime, the overall security situation in Baghdad has improved."
Most Iraqi cities no longer have a curfew.