SAN FRANCISCO  The Justice Department has begun an investigation into whether the recruiting practices of some of the largest technology companies violated antitrust laws, according to several people with knowledge of the investigation.

Companies including Google, Yahoo, Apple and Genentech have received formal requests for documents and information related to the inquiry, these people said. Antitrust lawyers said companies that receive such requests are not necessarily targets of an investigation.

The exact focus of the inquiry is unclear, but the people familiar with it said Justice Department lawyers appeared to be looking into whether the companies involved agreed to not actively recruit employees from each other.

Other companies that have received requests for information include Microsoft and Intel, according to these people, who agreed to speak on condition of anonymity because the investigation is confidential.