A senior US senator on Sunday urged an independent investigation of Scotland's decision to free the Lockerbie bomber, expressing concern that British interest in Libyan oil may have played a role. Senator Joe Lieberman pointed to 'shocking' suggestions by Libyan leader Moamer Kadahfi, his son Saif and the head of the British Libyan Business council that the release was mixed with Britain's interest in exploring oil in Libya. 'I don't want to believe that 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 by the Scottish justice minister to release a mass murderer,' he said on CNN. Abdelbaset Ali Mohmet al-Megrahi's return to a hero's welcome in Tripoli Thursday set off an outpouring of anger here over the Scottish government's decision to release him on compassionate grounds. Scottish Justice Secretary Kenny MacAskill said he made the decision on his own after doctors concluded that Megrahi, who suffers from prostate cancer, had just a few months to live. But adding fuel to the outrage was a claim by Seif al-Islam, Khadafi's son, that the issue of Megrahi's fate 'was always on the negotiating table' in talks with Britain over Libya's huge reserves of oil and gas. Senator Ben Cardin, a Democrat, said Lieberman's questioning of Scottish motives in releasing Megrahi 'raises a very valid point.' 'I think we need to know what this oil deal was all about and whether there was a compromise to the judicial system for commercial gain,' he said on the same CNN program. Both Lieberman and Cardin said Libya's celebratory homecoming for the former intelligence officer should have consquences. Lieberman called it 'a real setback for the anti-terrorist cause and takes our relations with Libya back to where they were for too long -- a bad place.' Lieberman was in Libya with a US delegation before Megrahi's release and had urged Tripoli to show restraint upon the Megrahi's return. 'We hoped the Colonel (Libyan leader Moamer Kadhafi) would get our message that he could not expect relations with the United States, which have been good since after the Iraq war of 2003 ... to go on normally if Megrahi was not only released but greeted as a hero,' he said. Lieberman, an independent who was a vice presidential candidate in 2000, noted that bilateral relations had been on the upswing in recent years, with Tripoli cooperating in destroying its weapons of mass destruction and aiding US counterterrorism efforts. Some of that progress, he said, had been dealt a blow. 'Don't expect President (Barack) Obama to meet Kadhafi at the UN General Assembly in New York in September,' he said, signaling one of the first areas where Libya is likely to feel Washington's cold shoulder. Cardin, calling Megrahi a mass murderer, said giving him a compassionate release 'was wrong.' 'I think it's very serious and I think there should be consequences,' he said. Meanwhile, Senator Richard Lugar, the top Republican on the Senate Foreign Relations Committee, tempered the outrage somewhat, telling CNN that while Washington 'ought to condemn as strongly as possible this release,' it was also necessary 'to continue our relations with Libya.' He suggested that political realities at home drove the reception received by Megrahi. 'I think it's important to notice that President Kadhafi has a constituency in Libya. And the rest of the world is now engaged in diplomatic relations with Libya,' Lugar said. Kadhafi met Friday with the Lockerbie bomber amid mounting Western outrage over the welcome he received upon his return. Libyan national television showed images of Kadhafi receiving and embracing Megrahi. Megrahi, the only person convicted for the December 1988 bombing of Pan Am Flight 103, which killed 270 people in the air and on the ground in the Scottish town of Lockerbie, was sentenced to life in prison in 2001. US President Barack Obama on Friday called the red-carpet reception Megrahi received 'highly objectionable', while his spokesman Robert Gibbs denounced the display as 'outrageous and disgusting.' 