Sardar Mohammad, who shot Ahmed Wali Karzai Tuesday in Kandahar, also attended regular meetings with U.K. officials, and two of his brothers-in-law worked in a CIA-run paramilitary unit, the newspaper said.
Mohammad's action was initially believed to be motivated by personal circumstances, but security officials soon learned Taliban intelligence operatives had infiltrated Afghan security and reached Karzai's innermost circle.
The insurgents "get these very big victories quite often and I think probably we underestimate the [Taliban's] intelligence components," one Western analyst told the newspaper.
"Our investigation shows that for the last three months he was acting out of character, not normal, erratic," said Mahmoud Karzai, another of Karzai's brothers.
"He wasn't sleeping, he was nervous, he was getting phone calls in the middle of the night, and our information shows he made a trip to Quetta [in Pakistan] and met with some Taliban. His father was a mullah. So all these things combined, plus the Taliban claim of responsibility ... but our preliminary investigation indicates this was the work of the Taliban."
The Washington Post reported Ahmed Wali Karzai met with Mohammad six days a week and had even taken his mother to Mohammad's house.
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