An investigation into the release of the Lockerbie bomber is to be broadened in an attempt to force Kenny MacAskill to stop “stonewalling” and give evidence, senior US senators have said.

The Senate’s foreign relations committee postponed a hearing into the furore, due to be heard on Thursday, after a series of key witnesses refused to appear.

Mr MacAskill, the Scottish Justice Minister, argued he had nothing to tell senators about alleged links between BP and the release as he had received no contact from the oil giant before making the decision.

But the Senate’s foreign relations committee accused him of “stonewalling” and said it was “pretty outrageous” he and other key witnesses had refused to appear....