Federal prosecutors overseeing the investigation into former President Donald J. Trump’s handling of classified documents have issued a subpoena for information about Mr. Trump’s business dealings in foreign countries since he took office, according to two people familiar with the matter. It remains unclear precisely what the prosecutors were hoping to find by sending the subpoena to Mr. Trump’s company, the Trump Organization, or when it was issued. But the subpoena suggests that investigators have cast a wider net than previously understood as they scrutinize whether he broke the law in taking sensitive government materials with him upon leaving the...