Two U.S. troops were killed Saturday by an Iraqi soldier who apparently smuggled real bullets into a training exercise and opened fire, raising fresh concerns about insurgents worming into the nation's security forces as the Americans prepare to leave by the year's end. A U.S. military official said the shooter was immediately killed by American soldiers who were running the morning drill at a training center on a U.S. base in the northern city of Mosul. The U.S. official said the exercise was not meant to involve live ammunition, and an Iraqi army officer said the shooting appeared to have been planned. Both officials spoke on condition of anonymity because they were not authorized to release the information. A U.S. statement confirmed that two soldiers were killed and a third was wounded by small-arms fire by what the military described as "an individual wearing an Iraqi army uniform." "This incident occurred during a training event being conducted by U.S. forces as part of their advise and assist mission with Iraqi security forces," the U.S. military said in a statement. The Americans were not identified pending notification of next of kin, and the statement provided few other details. The U.S. troops were from the 4th Brigade, 1st Cavalry Division, based at Ft. Hood, Texas. Additionally, another American soldier was killed Saturday during an unrelated military operation in central Iraq, making it one of the deadliest days for U.S. forces in the country in months. A U.S. military statement offered no details about that death. The Mosul attack underscores the threats that U.S. forces continue to face in Iraq even though most of the estimated 47,000 troops no longer go on regular combat missions. The vast majority of American troops left – down from nearly 170,000 in 2007 – are all but confined to bases where they help train Iraqi police, soldiers and pilots how to protect the country from threats like insurgents and invasions. Saturday's drill was designed to show security forces how to launch attacks and capture suspects, said an Iraqi military official, and it aimed to showcase U.S. training efforts before a Monday visit by top U.S. and Iraqi generals. Both nations have been eager to highlight Iraq's forces before U.S. troops leave the county at the end of the year after eight years of war, as required by a security agreement brokered in 2008 by Washington and Baghdad. Details about the deadly exercise were sketchy Saturday afternoon. A pair of Iraqi security officials said two assailants were captured after the shooting. The U.S. military official disputed that account. Such confusion is common immediately following a deadly event. Iraqi defense spokesman Maj. Gen. Mohammed al-Askari declined comment, saying he had not yet received an official report about the shooting. An officer with Third Iraqi Army Division, which was participating in the training, said real bullets had been banned from the drill – meaning the Sunni Muslim soldier smuggled in the ammunition with the intent to attack. The officer also spoke on condition of anonymity. Sunnis formed the backbone of the insurgency in Iraq, and the fact that the soldier who opened fire on the Americans was Sunni reinforces suspicion the attack may have had an insurgent link. Attacks by Iraqi soldiers and police against Americans are not unprecedented, especially in Mosul, a former al-Qaida haven and Iraq's third-largest city, located 225 miles (360 kilometers) northwest of Baghdad. In May 2009, two U.S. soldiers were killed and three were wounded in Mosul by a gunman wearing an Iraqi army uniform. Three months earlier, two Iraqi policemen opened fire on U.S. soldiers and Iraqi police who were lunching at a Mosul police station. The attack killed a U.S. soldier and his interpreter. Additionally, two U.S. soldiers were killed and six wounded Nov. 12, 2008, when an Iraqi trooper opened fire on them in an Iraqi outpost in Mosul. The Iraqi was killed. And just one week after the U.S. military triumphantly announced it had withdrawn all but 50,000 troops to mark the end of the U.S. combat mission in Iraq, two American troops were killed by an Iraqi soldier during a visit to an Iraqi army base near Kirkuk, also in northern Iraq. The American soldiers were at the base to protect an Iraqi military commander. Deaths among American service members have fallen dramatically since the deadliest days of the war. On Sept. 24, three soldiers involved in the Iraq combat mission died in separate noncombat incidents, but one was in Kuwait. Four U.S. service members were killed by hostile fire in September 8, 2009.