Al-Mujamai was a senior member of the Mujamai tribe, one of the largest in Diyala.
The targeted killings of policemen, soldiers, government officials and former fighters are stoking tensions in Iraq following a parliamentary election in March that produced no clear winner and has yet to yield a government.
Seven other people were killed in similar attacks in Baghdad, Mosul and Fallujah. The Fallujah attack killed Najim Abid al-Issawi, a Sahwa member and vocal critic of al-Qaeda in Iraq.
Series of attacks
The killings in Buhriz are the latest blow in what appeared to be a campaign to undermine Iraq's fragile security gains.
Last week, armed men shot dead a local council leader and four members of his family in Falluja, west of Baghdad.
Police blamed the killing on al-Qaeda.
The Awakening Councils are seen as crucial to defending significant security gains made over the past few years.
Iraqi leaders promised to give government jobs to some 90,000 Sahwa members, but only about 42,000 of the fighters have been offered jobs, according to government officials.
Thousands of other Sahwa fighters were asked to stay with their neighbourhood security patrols through Iraq's March 7 parliamentary election and until a new government is formed.
The Awakening Councils were credited with helping to significantly cut violence since the worst of the sectarian bloodshed in 2006-07, when tens of thousands of people were killed.
On May 12, a minivan packed with explosives blew up at a crowded market in the town of Khalis, also in Diyala, killing at least 30 people and wounding 80 others.
Before joining the fight against al-Qaeda, members of the Awakening Councils were accused of killing American and Iraqi soldiers.
Some of their former leaders and fighters were arrested by Iraqi security forces to face those accusations, forcing others into hiding.
Many other Sahwa leaders have been targeted by bombings and assassinations in recent weeks in what appear to be either acts of revenge or part of a campaign to scare them back into the ranks of the uprising.