If a contractor is unable to support any part of his claim and it is determined that such inability is attributable to misrepresentation of fact or fraud on the part of the contractor, he shall be liable to the Government for an amount equal to such unsupported part of the claim in addition to all costs to the Government attributable to the cost of reviewing said part of his claim. Liability under this subsection shall be determined within six years of the commission of such misrepresentation of fact or fraud.