Gunmen ambushed a police convoy transporting three alleged kidnappers to a prison in northern Mexico, killing seven officers and freeing two of the suspects, authorities said Saturday. Two state police officers were wounded in the attack Friday and the third alleged kidnapper was flown to Mexico City where he is being questioned by organized crime investigators, the federal attorney general's office said in a statement. It said those killed were six local police officers and a traffic officer. Police had arrested the three alleged kidnappers and killed a fourth earlier Friday after stopping a truck with Texas plates. As they transported the three men to a prison in the northern city of Zacatecas, a group of gunmen opened fire on the convoy. The attack took place about 50 kilometers south of the city.