The man, who is not being named for security reasons, said he was spared after saying he was an Afghan. "Where are the foreigners?" they shouted.
Fourteen foreigners are confirmed to have died, along with four Afghans.
The interior minister said the recent decision to transfer security to a private company had been a mistake.
Some 160 people were rescued after Afghan troops fought throughout the night to regain control of the building.
Three attackers were also killed in the siege, officials said. Ten people, including four civilians, were injured.
