"My son died for the sins of George Bush and Donald Rumsfeld. This administration did this," Michael Berg said in an interview on Thursday.
Five masked men appeared to saw off Nick Berg's head and the video of his decapitation was posted on an Islamist website on Tuesday.
Michael Berg said his 26-year-old son, a civilian contractor, probably would have felt positive, even about his executioners, until the last minute.
"I am sure that he only saw the good in his captors until the last second of his life," he said. "They did not know what they were doing. They killed their best friend."
He also attacked the Bush administration for its invasion of Iraq and its sponsorship of the Patriot Act, which gives sweeping powers of surveillance to the federal government.
"My son died for the sins of George Bush and Donald Rumsfeld. This administration did this"
Father of slain Nick Berg
The father had earlier blamed US-occupation forces of being responsible for Nick's death by detaining him in Iraq.
The detention, Michael said, derailed Nick's plans to return early to the US, ultimately costing him his life.
Michael Berg rejected US government claims that his son had never been held by American authorities in Iraq.
"I have a written statement from the State Department in Baghdad … saying that my son was being held by the military," he said. "I can also assure you that the FBI came to my house on 31 March and told me that the FBI had him in Mosul in an Iraqi prison."
Looking for work
A communications contractor looking for work in occupied-Iraq, Nick Berg planned to return to the US by the end of March. But he was detained in Iraq on 24 March and released only on 6 April.
He subsequently went missing from 9 April – a chaotic period when dozens of foreigners were abducted by Iraqi fighters.
Michael Berg filed a lawsuit against the US government on 5 April, naming Rumsfeld and alleging his son was being held illegally by the US military in Iraq. The next day, his son was released.
Nick Berg's decapitated body was discovered by a road near Baghdad, three days before the gruesome video of his killing surfaced.