Federal criminal investigators looking into the collapse of Countrywide Financial Corp. have been calling witnesses before a grand jury, say people familiar with the matter. Such a step suggests that the investigation of the one-time mortgage giant, which has been continuing for about two years, could be moving closer to a resolution. The grand jury began hearing witnesses on the Countrywide case late last year, say people familiar with the matter. Word of this development hasn’t yet surfaced publicly, since grand-jury proceedings are routinely shrouded in secrecy. It isn’t known which individuals the Countrywide grand jury here is looking at or what potential crimes are being investigated. Last June, the SEC filed a civil suit in Los Angeles federal court against three former top Countrywide executives, including the company’s longtime chief executive, Angelo Mozilo.