SoftBank Group’s acquisition of semiconductor designer Ampere Computing is facing a potentially lengthy probe by the U.S. government.

The Federal Trade Commission, one of two regulators charged with reviewing deals, has opened an in-depth investigation of the takeover, known formally as a second request for information about the transaction, according to people familiar with the matter who asked not to be identified because it wasn’t public yet.

Only a small portion of deals face such follow-up requests. In some instances investigations can last for a year or more and are a precursor to a lawsuit to block the deal.