Sardar Mohammad, a police commander, had given the U.S. military information used to arrest many Taliban fighters, three of his relatives told The Washington Post Thursday.
On Tuesday, he shot and killed Ahmed Wali Karzai, a powerful figure in Kandahar. The Taliban has claimed responsibility for the assassination.
A senior Afghan official said the Taliban somehow won over Mohammad, and he may have been a longtime sleeper agent.
Mahmood Karzai, another brother of the president, said Mohammad met with Taliban leaders in Quetta, Pakistan, within the past three months and began acting strangely, sleeping poorly and changing houses at night.
The Afghan official could not confirm the Quetta trip but said Mohammad, 35, had attended a Pakistani madrassa in his youth.
Mohammad's family firmly denied he would have joined the Taliban and said he had not been in Pakistan for 20 years. They said he was devoted to the Karzais, and a family member helped dig Ahmed Wali's grave.
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