Conrad Black, once a media mogul whose newspaper empire spanned several continents, is headed back to prison after a federal judge ruled Friday that he had not served enough time for defrauding investors.

Judge Amy St. Eve sentenced Black to 3 1/2 years in prison after berating and then praising him. But prosecutors say he will be given credit for more than two years he already had served, meaning the 66-year-old will go back for a little more than a year.