An Iraqi truck driver who failed to stop when signaled on a main highway north of Baghdad was shot and killed early Friday by an American soldier, a spokesman for the U.S. military said. 'This is a tragic and regrettable incident, but the soldier perceived a credible threat,' the spokesman, Maj. Derrick Cheng, said in an e-mail message. Such killings were common during much of the Iraq war but have become relatively rare. The shooting happened on a stretch of the highway between Balad and Tikrit, in Salahuddin province, a predominantly Sunni Arab area. Cheng said a logistics convoy had stopped on the highway about 2:15 a.m. after one of its vehicles broke down. As an Iraqi truck approached, he said, the soldiers signaled for it to stop. When it did not, he said, a soldier opened fire, killing the driver. A teenage passenger in the truck was not injured. Cheng did not immediately respond to questions about what kind of signal the soldiers used. The shooting was not widely reported in Iraq on Friday. Cheng said that the killing, which came 10 days after the official withdrawal of U.S. combat forces from Iraqi cities and towns, was being jointly investigated with the Iraqis. Under the terms of the security agreement signed by Iraq and the United States last year, Iraq has jurisdiction in situations involving U.S. soldiers only when they are off duty and acting outside their bases. Even then the United States retains custody of all U.S. soldiers and military contractors suspected of wrongdoing. 