US senators doggedly investigating the Lockerbie bomber's release one year ago hope an ever-wider dragnet will reveal whether embattled oil giant BP improperly shaped the widely condemned decision.

Frustrated by what they have condemned as "stonewalling" by BP and the British and Scottish governments, the lawmakers have even appealed to informants to share behind-the-scenes secrets on the official deliberations.

The goal: Get as much information as possible before the US Senate Foreign Relations Committee holds a public hearing in September on the forces that shaped the decision to free Libyan national Abdelbaset Ali Mohmet al-Megrahi.

Democratic Senators Robert Menendez -- who plans to chair the hearing -- and Frank Lautenberg of New Jersey as well as Chuck Schumer and Kirsten Gillibrand of New York have spearheaded the election-year drive to get to the bottom of what the British government has acknowledged as "a mistake."

Scotland's devolved government freed Megrahi over US objections on August 20, 2009 on grounds he was suffering from terminal cancer and had only three months to live -- but he is still alive nearly a year later.

Megrahi, who received a warm welcome upon arriving in Libya, is the only man convicted of the 1988 bombing of Pan Am Flight 103 over the Scottish town of Lockerbie, in which 270 people died, including 189 Americans.

The US Senate hearing, postponed from late July, is set to examine whether BP had improperly lobbied for Megrahi's release in order to safeguard a 900-million-dollar oil exploration deal with Libya.

The oil giant as well as British and Scottish officials have denied the charge, which has roiled transatlantic ties, and it was unclear whether any of the parties would send representatives to the hearing.

Scotland's First Minister Alex Salmond said recently that his ministers would be willing to meet with US lawmakers in Britain, but that there was "no way on Earth" that the US committee would hold a hearing in London or Scotland.

Salmond also expressed anger at the continued US pressure for Scotland to send Scottish Justice Secretary Kenny MacAskill to the Senate hearing, saying American ministers would never bow to such demands from another country.

"You shouldn't ask other people to do things that your own government would never dream of in history," said Salmond.

US Senate criticism of the decision started almost immediately after Megrahi was freed, with a unanimous September 2009 vote to condemn his release as well as what lawmakers in Washington denounced as his "hero's welcome" in Libya.

In July 2010, with BP under heavy fire over the disastrous Gulf of Mexico oil spill, senators made a fresh push for an investigation into whether the oil giant had improperly lobbied for the bomber's freedom.

On July 15, Britain's ambassador to the United States, Nigel Sheinwald, said freeing Megrahi was "a mistake" and said London "deeply regrets the continuing anguish" the decision caused the families of those killed.

Not long after, the foreign relations committee scheduled its hearing -- only to postpone it when the governments involved and BP declined to send key witnesses.

On July 16, US Secretary of State Hillary Clinton told British counterpart William Hague that each day Megrahi is free in Libya was an "affront" to the families of the victims, her spokesman said.

At US President Barack Obama's side just days later, British Prime Minister David Cameron says he has ordered a new review of the file, but says there is no need for a full re-investigation and denies officials were "swayed by BP."

"I don't think there's any great mystery here. There was a decision taken by the Scottish Executive, in my view a wholly wrong and misguided decision, a bad decision, but their decision nonetheless," said the prime minister.

Cameron, making his first visit to Washington since taking office, also makes a dramatic 11th-hour U-turn, agreeing to meet with the US senators after saying he did not have time.

Schumer emerged from the meeting with Cameron saying a new probe was "not off the table."

Over the next month, the US lawmakers renewed their call for key witnesses and, this week, promised to keep secret the identities of anyone providing behind-the-scenes information about the decision.