Armed men in several cars stopped the bus around 2 p.m, as it was carrying workers from the Mosul Textile Factory to their hometown of Bashika, which has a mixed Christian and Yazidi population. The gunmen checked passengers' identification cars, then asked all Christians to get off the bus, said police Brig. Mohammed al-Wagga.
They hijacked the bus with all the Yazidis still inside, and drove them to eastern Mosul, where they were lined up along a wall and shot to death execution-style, al-Wagga said.
A police spokesman for Ninevah province, where Mosul is the provincial capital, said the executions were in response to the killing two weeks ago of a Yazidi woman who had recently converted to Islam.
Yazidis are a small group concentrated mostly around the northern city of Mosul, 225 miles northwest of Baghdad. They are primarily Kurdish, and worship an angel figure that some Muslims and Christians consider the devil.
The woman had fallen in love with a Muslim man, then converted to Islam and ran off with him, said police spokesman Abdul-Karim Khalaf. Her relatives disapproved of the match and dragged her back to Bashika, where she was stoned to death, he said.
Voice of Iraq reported on the death earlier this month, quoting eyewitnesses who claimed that 2,000 people took part.
A grainy video showing gruesome scenes of the woman's killing was distributed on Iraqi Web sites in recent weeks, but its authenticity could not be independently confirmed.
Sunday's killings by Muslim extremists were an attempt to avenge the woman's death, Khalaf said.
About a week ago, the Ninewa police chief had ordered the arrest of people responsible for the stoning. It was not known if any arrests had occurred.
After the killings, hundreds of Yazidis took to the streets of Bashika. Shops were shuttered and many Muslim residents closed themselves in their homes, fearing reprisal attacks. Police set up additional checkpoints across the city.
Bashika is about 80 percent Yazidi, 15 percent Christian and about five percent Muslim.