Basically the US senate is saying BP used there influence to get the Libyan conviced of the lockerbie bombing released back to Libya, a charge both the UK and Scotish governments deny.
The scots have jurisdiction here and released him on human rights grounds (he has terminal cancer).
The Senate also want a number of British & Scotish politicians to go to washington to explain the decision, something they have all decided not to do as, in there own words 'it is an event that is not in US juridiction' and fair play to them after all would a US politician goto a commitie in another country if they were asked?