The Justice Department is trying to determine whether Google Inc. would gain too much sway over the online travel industry by acquiring ITA Software Inc., which powers the Web's most popular airline-ticket search and booking sites, said people familiar with the department's review.

The merger investigation is at an early stage, but according to people familiar with the situation, Justice antitrust authorities are focusing on two potential areas of concern: whether rivals would continue to have access to ITA's data and whether...