Adm Mike Mullen said it was an "obviously political" decision
The top US military officer has joined in condemning Scotland's release of Lockerbie bomber Abdulbaset al-Megrahi.
Speaking on US network CNN's State of the Union, chairman of the Joint Chiefs of Staff Adm Mike Mullen said: "This is obviously a political decision."
On the same programme, influential Senators Joe Lieberman and Ben Cardin questioned whether the move had been made to improve British-Libyan trade.
Scotland's First Minister Alex Salmond has said it was the "right decision".
Terminally-ill cancer patient Megrahi, 57, was freed last week on compassionate grounds.
He had been given a life sentence in 2001 after being found guilty of killing 270 people in the 1988 Pan Am Flight 103 bombing.
The Libyan returned home to Tripoli to jubilant scenes that included people waving Scottish flags.
American anger over the decision was voiced on television programmes on Sunday.
Mr Lieberman, an independent senator who called for an investigation into the release, said it was a "real setback for the anti-terrorist cause".
He continued: "It takes our relations with Libya back to where they were for too long - a bad place.
Megrahi was given a hero's welcome by Col Gaddafi's son
"The suggestions that have followed - that there was an intermixing of Megrahi's fate with British interests and oil exploration in Libya - are shocking.
"I don't want to believe they are true, but they are hanging so heavily in the air that I hope that our friends in Britain will convene an independent investigation of this action to release a mass murderer."
He was echoed by Democratic Senator Ben Cardin, who said: "The terrorist shows no compassion and to give him a compassionate release was wrong.
"I think it's very serious and I think there should be consequences."
Mr Cardin, a Democrat, added: "We need to know what this oil deal is all about and whether there was a compromise to the judicial system for commercial gain."
The latest comments come after the head of the US Federal Bureau of Investigation launched his own scathing attack on Scottish government over Megrahi's release.
In a letter sent to the Scottish justice secretary, Robert Mueller said the decision had made "a mockery of justice" and given "comfort to terrorists around the world",
Defending the decision, Alex Salmond told the BBC: "We understand the upset. We understand the disagreement. But we have to do what is right in terms of our legal system, what we are duty-bound to do.
"No-one seriously believes that we made any other decision except for the right reasons."
The Scottish government can take decisions independently of London on justice matters but not foreign affairs.