Officials say Ahmed Wali Karzai was killed at his home in southern Afghanistan on Tuesday. A person who witnessed the killing said a member of Wali Karzai's private security team shot him.
Tooryalai Wesa, the provincial governor of Kandahar, identified the assassin as Sardar Mohammad and said he was a close, "trustworthy" person who had gone to Wali Karzai's house to get him to sign some papers.
As Wali Karzai was signing the papers, the assassin "took out a pistol and shot him with two bullets - one in the forehead and one in the chest," Wesa said.
The killing coincided with a visit to the capital, Kabul, by French President Nicolas Sarzoky.
"This morning my younger brother Ahmed Wali Karzai was murdered in his home," the Afghan president said during a joint news conference with Sarkozy. "Such is the life of Afghanistan's people. In the houses of the people of Afghanistan, each of us is suffering and our hope is, God willing, to remove this suffering from the people of Afghanistan and implement peace and stability."
Wali Karzai was the head of the Kandahar provincial council. He had been seen as a liability for his half brother's government after a series of allegations including that he was on the CIA payroll and involved in drug trafficking.
His death leaves a dangerous power vacuum in the south just as the government has begun peace talks with insurgents ahead of a U.S. withdrawal.
The Associated Press contributed to this story.