Relatives discovered the body of a kidnapped Iraqi policeman on Thursday, but it was rigged with explosives and blew up, killing his father and brother, police and a doctor said.
The policeman was kidnapped in a village near Sharqat, northwest of Baghdad, the previous day. The blast also wounded a second brother.
Militants in Iraq generally carry out car bombings or bury devices in or near roads, but more rarely rig bodies and even animals such as donkeys with explosives for use in attacks.
Elsewhere on Thursday, a car bomb exploded in northern Baghdad, killing two people and wounding at least eight, officials said.
The violence comes as officials tally votes from Iraq's April 30 elections, amid a protracted surge in bloodshed that the government has blamed on external factors such as the civil war in neighbouring Syria.
But analysts and diplomats say widespread anger in the minority Sunni Arab community over alleged mistreatment at the hands of the Shiite-led authorities has played a major role in the violence.
More than 3,100 people have been killed already this year, according to AFP figures based on security and medical sources.
The unrest is the worst since Iraq emerged from brutal Sunni-Shiite sectarian fighting that peaked in 2006-2007 and killed tens of thousands of people.