The U.S. military has closed an investigation into the January killings of four Iraqis whose deaths were blamed on American troops, saying it couldn't find the culprits, the Army said today. No one claimed responsibility for the Jan. 3 deaths of the Iraqis — a taxi driver and three passengers, including a woman and a 7-year-old boy — who were killed by heavy machine-gun fire as their cab traveled near Tikrit. The sole survivor, another passenger, blamed American troops.