KABUL—The security aide who murdered Afghan President Hamid Karzai's half-brother on Tuesday was a Taliban recruit who embraced the insurgency three months ago, another Karzai sibling said.

Immediately after the slaying of Ahmed Wali Karzai, the Kandahar provincial council chief, his associates and Afghan and U.S. officials described the assassination as caused by personal grievances and unrelated to the Taliban.

But since then, the Afghan investigation has shown that Sardar Mohammed, the police commander who gunned down Ahmed Wali Karzai, recently met with Taliban officials in Quetta, Pakistan, said Mahmood Karzai, the president's older brother.

"We are investigating that he was in Quetta during the last three months," Mahmood Karzai said in an interview. "There are a lot of young people that only know war, and it's very easy for manipulators to turn our own people against us."

The confirmation of the Taliban connection to the assassination, and Thursday's deadly bombing of the mourning ceremony for Ahmed Wali in Kandahar, illustrated the insurgency's power and reach in southern Afghanistan, despite the U.S. troop surge in the region. Mr. Sardar's links with the insurgency were reported Friday by the Washington Post.