Monday's attack came amid a security crackdown in Mosul and nearby areas by Iraqi forces, backed by the US military, to root out al-Qaeda in Iraq fighters.
Those fighters have carried out frequent attacks against police recruits, killing hundreds.
The commander of US forces in northern Iraq said joint security forces had arrested about 100 suspects thought to be crucial to the operations of armed groups in their offensive in the region.
Al-Qaeda in Iraq and supporting organisations were believed to have regrouped in Nineveh after being pushed out of other parts of Iraq.