It is not unusual for companies to seek information from former employees regarding that employee's tenure with the company. Sometimes the exchange is very minimal and informal, while other times the initial contact is a precursor to a more formal exchange and potentially a deposition.

Without knowing the context of the request, since the Department of Justice is involved, it would probably be a good idea hire an attorney to represent you. Getting to the core of your question (what type of lawyer), ideally you should look for an attorney that has both subject matter expertise and the appropriate background / experience. If you know (or can guess) the subject matter of the investigation, it would be a good idea to seek a litigator (not a transactional or regulatory attorney) that has a background in that subject matter.  If you are unaware of the subject matter, it may be a good idea to seek counsel from a large firm that can handle both employment and white collar criminal matters.