WASHINGTON—The Justice Department is conducting a wide-ranging antitrust investigation into whether cable companies are acting improperly to quash nascent competition from online video, according to people familiar with the matter.

Justice Department officials have spoken to several online video providers, including Netflix Inc. and Hulu LLC, those people said. Investigators have also questioned Comcast Corp., Time Warner Cable Inc. and other cable companies about issues such as setting data caps, limits to the amount of data...