UK Prime Minister David Cameron has insisted BP should not be blamed for the "completely wrong" decision to release the Lockerbie bomber.
Claims have been made that BP lobbied for the release, but Mr Cameron said the Scottish government was responsible for freeing Abdelbaset Ali al-Megrahi.
Mr Cameron has asked the UK's top civil servant to review government papers but ruled out US demands for an inquiry.
The Scottish government has denied any BP influence in the release last year.
Standing alongside US President Barack Obama, Mr Cameron said he had seen no evidence the Scottish government - which made the decision to free terminally ill cancer patient Megrahi on compassionate grounds - had been "swayed" by lobbying from BP.
The firm is already facing widespread criticism in the US for its handling of the enormous oil leak from one of its platforms in the Mexican Gulf.
BP would have to explain any representations it had made over Megrahi, Mr Cameron said at a White House press conference after a three-hour meeting with the US president.
He and President Obama had been in "what we called a violent agreement" that freeing the bomber last August had been an error, he added.
Mr Cameron, on his first official trip to the US since becoming prime minister, said: "Releasing the Lockerbie bomber, a mass murderer, was completely wrong.
"He showed his victims no compassion. They were not allowed to die in their beds at home."
But he added: "That was not a decision taken by BP, it was a decision taken by the Scottish government."
Mr Obama said people in the US had been left "surprised, disappointed and angry".
He told reporters he would welcome new information on why the Scottish government, which has responsibility for justice issues in Scotland rather than the UK government, had freed Megrahi.
US Secretary of State Hillary Clinton has called for an inquiry into how the decision was reached.
But Mr Cameron said: "I don't think there's any great mystery here."
He added: "I don't need an inquiry to tell me it was a bad decision. It was a bad decision."
Later, the prime minister met four senators from New York and New Jersey, where many of the bombing victims came from, who are pressing for an investigation into the bomber's release.
The UK government had initially ruled out a meeting with the senators, before a change of plan.
A Downing Street spokesman said: "The prime minister recognises the strength of feeling and knows how important it is to reassure the families of the victims.
"We are happy to see them face to face and find time in the diary."
Some 270 people died in the 1988 bombing of a Pan Am flight over Scotland, most of them Americans.
The bomber was freed by Scottish Justice Minister Kenny MacAskill and allowed to go home to Libya.
Megrahi has prostate cancer, and at the time he was said to have as little as three months to live. He is still alive almost a year later.
The Scottish government said the decision to free Megrahi had been "based on strict justice criteria and no other factor".
Mr MacAskill said he could give "an absolute assurance that BP did not contact myself or the Scottish government and it formed no part of my decision".
Scottish First Minister Alex Salmond said: "'I'm not a doctor but I think it's entirely possible that somebody's life expectancy in a prison in Greenock is somewhat shorter than the life expectancy with aggressive drug therapy in Tripoli.
"It doesn't alter the fact that Mr Megrahi has terminal cancer and will die because he has terminal cancer."
BP has insisted it had no discussions with either the UK or Scottish governments over the issue.
During the White House press conference, Mr Obama described the US's relationship with the UK as "truly special".
He and Mr Cameron said it was important the two countries continued to work together in Afghanistan and to ensure the recovery of the world economy.
The UK has invited Mr Obama to make a state visit, but it is unlikely this will happen until next year at the earliest.