Abu Risha and two of his bodyguards were killed by a roadside bomb planted near the tribal leader's home in Ramadi, Anbar's provincial capital, said Col. Tareq Youssef, supervisor of Anbar police.
No group claimed responsibility for the assassination but suspicion fell on al-Qaida in Iraq, which U.S. officials say has suffered devastating setbacks in Anbar thanks to Abu Risha and his fellow sheiks. It's unclear how his death would affect U.S. efforts to organize Sunnis against the terror network.
Abu Risha was among a group of tribal leaders who met U.S. President George W. Bush earlier this month at al-Asad Air Base in Anbar province.
But privately, two officials said his assassination would be a huge setback for U.S. efforts in Iraq, because it sends a message to others who are cooperating with coalition forces or thinking about cooperating against al-Qaida.
A Ramadi police officer said Abu Risha had received a group of poor people at his home earlier in the day, as a gesture of charity marking the beginning of the Muslim holy month of Ramadan. The officer, speaking on condition of anonymity out of security concerns, said authorities believed the bomb was planted by one of the guests.
A senior member of Abu Risha's group, Sheik Jubeir Rashid, called the assassination a "criminal act" and blamed al-Qaida.
"It is a major blow to the council, but we are determined to strike back and continue our work," Rashid said. "Such an attack was expected, but it will not deter us."
After the bombing, police announced a state of emergency in Ramadi and set up additional checkpoints throughout the city, Rashid said.
Interior Ministry spokesman Maj. Gen. Abdul-Karim Khalaf said that after the first blast which killed Abu Risha, a car bomb exploded nearby.
"The car bomb had been rigged just in case the roadside bomb missed his convoy," Khalaf said. There were no casualties from the car bomb, he added.
The Interior Ministry swiftly ordered plans for a monument built to honor Abu Risha as a "martyr," Khalaf said. It would be build either at the explosion site, or at the center of Ramadi, he said.
Anbar police were investigating the attack, and the Interior Ministry would send a committee to assist, Khalaf added.
It was not the first time Abu Risha and his colleagues have been targeted.
In June, a suicide bomber blew himself up in the lobby of Baghdad's Mansour Hotel during a meeting of U.S.-linked Sunni tribal leaders, killing 13 people and wounding 27. Among those killed was the former governor of Anbar and sheik of the al-Bu Nimir tribe, Fassal al-Guood - a key ally of Abu Risha. A day later, al-Qaida in Iraq claimed responsibility for the attack.