The bodyguard who assassinated President Hamid Karzai's brother had been working closely with United States Special Forces and the CIA before he was recruited by the Taleban.

That has raised fears over its increasingly sophisticated intelligence apparatus which has managed to threaten the inner circles of power in Afghanistan.

Sardar Mohammad, who shot Ahmed Wali Karzai at his home in Kandahar City last week, also held regular meetings with British officials, and had two brothers-in-law serving in a CIA-run paramilitary unit, the Kandahar Strike Force, the Washington Post reported.

Yet evidence is emerging that the Taleban recruited Mohammad - who was believed to be a friend, confidant and trusted lieutenant of Ahmed Wali Karzai - in an infiltration of the Afghan Government's security apparatus.

"Our investigation shows that for the last three months he was acting out of character, not normal, erratic," said Mahmoud Karzai, another brother.

"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 Taleban. His father was a mullah. So all these things combined, plus the Taleban claim of responsibility ... but our preliminary investigation indicates this was the work of the Taleban."

Security analysts say that, if true, it shows not only the problems facing the Afghan army and police as they start taking control of the country from Nato, but also how sophisticated Taleban intelligence operations have become.

The immediate assumption after Ahmed Karzai's death was that Mohammad was pursuing a personal vendetta, largely because the notion of defection to the Taleban was so hard to credit.

The insurgents "get these very big victories quite often and I think probably we underestimate the [Taleban's] intelligence components," said one Western analyst.

"They do have dedicated intelligence officers. And that's not just about gathering information but also about infiltration, using whatever combination of blackmail or ideological levers [they need to ...

"The killing] is a really excellent indication of the sophistication of Taleban intelligence networks. It's something we don't know enough about - how it breaks down," the analyst said.

But what makes Mohammad's defection so remarkable is just how close he was to Ahmed Wali Karzai. The Washington Post says he met the Kandahar strongman six days a week.

Ahmed Wali Karzai would pay the salaries of his policemen if the Government was in arrears, and had taken his mother to Mohammad's house.