WASHINGTON—The Justice Department is stepping up its investigation into hiring practices at some of America's biggest companies, including Google Inc., Intel Corp., International Business Machines Corp., Apple Inc. and IAC/InterActiveCorp., people familiar with the matter said.

The inquiry is focused on whether companies, particularly in the technology sector, have agreed not to recruit each others' employees in ways that violate antitrust law. Specifically, the probe is looking into whether the companies' hiring practices...