Gunmen broke into the home of a Kurdish policeman in the northern Iraqi city of Kirkuk early Sunday, killing his wife and three children execution-style with shots to the head as they slept, authorities said.

Omed Abdul-Hamid had already left for work when the gunmen burst into the house and shot the woman and children as they lay sleeping together on a mattress on the floor, police Brig. Gen. Sarhad Qadir said.

In vivid crime scene photos released by police, a 2-year-old boy could be seen lying on his back to the right of his mother with his head on a pillow, his face caked with blood where he had been shot. The boy's two sisters, ages 6 and 9, lay to the left of their mother; all three were face down and shot in the back of the head.

Abdul-Hamid was one of the tens of thousands of Kurds who were forced out of the area under Saddam Hussein in the government's massive resettlement effort to make the region Arab, Qadir said.

He, like scores of others, returned after the U.S.-led invasion in 2003, and there is now an uneasy coexistence of about 850,000 Kurds, Arabs and Turkomen — an ethnic Turkish minority — in Kirkuk.

The oil-rich city of Kirkuk has been at the center of recent tensions as the Kurds push to incorporate the city into their self-governed Kurdish region to the north despite opposition from Arabs and the Turkomen.

U.S. commanders have warned that insurgents are trying to exploit the tension to stoke an Arab-Kurdish conflict, which they say poses a greater long-term threat to Iraq's stability than the Sunni-Shiite conflict that nearly plunged the country into a civil war in 2006 and 2007.

The attack on Abdul-Hamid's family was likely carried out by insurgents, as nothing in the home was stolen, Qadir said. He said preliminary indications are that three gunmen took part in the attack.

Elsewhere in the area, two separate police patrols were hit with roadside bombs in southern Kirkuk, wounding six policemen, police Brig. Gen. Khatab Omar said.

Also in northern Iraq, four people were killed in an area about 60 miles (100 kilometers) south of Mosul in a tribal dispute, police said.