Unidentified gunmen targeted al-Lahibi in the city of Baquba and shot him to death Monday, the news website quoted sources as saying, adding one of al-Lahibi's sons also was shot in the attack but survived.
Police said they have opened an investigation.
The Sahwa forces, an unofficial and non-governmental military force, have been in Diyala province since 2007 and are regarded as a support agency for security forces facing al-Qaida, the website said.
Lahibi is among the hundreds of Iraqis who have been killed and wounded in recent weeks in attacks that have raised fears the country may fall into civil war, officials say.
Targets have included civilians and government workers as well as prisons, security forces and Sunni paramilitary groups fighting al-Qaida, China's official Xinhua News Agency reported Monday.
Responsibility for some of the attacks has been claimed by members of al-Qaida in a Mesopotamian militant group, capitalizing on protests by Sunni groups against the Shiite-led government.
The lack of unity within the Iraqi leadership has led to discontent around the country, said Sabah al-Sheik, a professor of politics at Baghdad University.
Sunni and Shiite extremist groups have carried out the attacks, "exploiting the deterioration of the political atmosphere as well as the sectarian division in Iraq and in the region as well," he said.
"Nobody cares about the public interest," lamented Jawad al-Hasnawi, a member of the al-Ahrar parliamentary bloc. "We have to admit that we all lack the culture of accepting the others, and with such a gloomy atmosphere, I can't see any solution on the horizon."
Wathiq al-Battat, the leader of a Shiite party, last week announced his intention to form a militia to help the security forces fight al-Qaida members and other extremists.
The Iraqi Interior Ministry responded by banning anyone not in law enforcement or members of the military from carrying weapons.