The bodyguard who assassinated President Hamid Karzais brother had been working closely with US Special Forces and the CIA before he was recruited by the Taliban, raising fears over the Islamist movements 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 on Tuesday, 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 yesterday.

Yet evidence is emerging that the Taliban recruited Mohammad – who was believed to be a friend, confidant and trusted lieutenant of Ahmed Wali Karzai – in an infiltration of the Afghan governments security apparatus.

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

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 Taliban intelligence operations have become.

The immediate assumption after Ahmed Karzais death was that Mohammad was pursuing a personal vendetta, largely because the notion of defection to the Taliban was so hard to credit. But that seems to be what happened. The insurgents "get these very big victories quite often and I think probably we underestimate the [Talibans] intelligence components," said one Western analyst.

"They do have dedicated intelligence officers. And thats 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 Taliban intelligence networks. Its something we dont know enough about – how it breaks down," the analyst said.

There has been a string of high-profile attacks by the Taliban against Afghan government officials, and a number of instances of agents infiltrating the Afghan security forces and killing Afghan or Nato troops.

On 28 May, General Daud, the top police commander in northern Afghanistan, was killed by a bomb as he met Nato officials. In Kandahar, the deputy governor and the chief of police were assassinated earlier this year.

But what makes Mohammads 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 Mohammads house.