Four teenage gunmen with concealed pistols carried out a raid on the luxury Serena hotel on Thursday, just weeks before Afghanistan votes for a successor to President Hamid Karzai.
The Taliban claimed responsibility but the Afghan authorities have pointed the finger across the border to Pakistan, saying on Sunday the attack was planned "outside the country".
Pakistan was the main supporter of the former Taliban regime in Afghanistan, and Afghan officials have long voiced suspicions about connections between the hardline movement and Islamabad's powerful intelligence services.
The NDS said a number of foreigners "related to intelligence services" were seen in the Serena taking suspicious pictures ahead of Thursday's attack.
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"NDS investigations and findings after the tragic incident reveal that Pakistani intelligence services were involved in planning this heinous attack," the agency said in a statement.
Pakistani foreign ministry spokeswoman Tasneem Aslam on Monday rejected the earlier Afghan claim of involvement, calling it "highly disturbing".
Seven senior employees of Shield Security, the company charged with guarding the Serena, have been detained over the incident, the Afghan interior ministry said.
Ahmad's youngest son Abuzar, who was hit in the head with a bullet fragment in the attack, remained in hospital today but doctors said his condition was showing signs of improvement.