A string of bombs targeted Iraqi army patrols and a police crime lab Tuesday in northern Iraq , killing at least four people, authorities said. The bombings were the latest attacks against security forces in Mosul, an area where insurgents retain a foothold despite a sharp drop in violence across the rest of the country. Iraqi forces frequently have been targeted by insurgents seeking to derail public confidence in the government. Security lapses that allowed for large and deadly attacks against a number of government buildings also have raised concerns about the Iraqis' ability to take over their own security so American troops can withdraw by the end of 2011 as scheduled.