Officials vowed to "seek redress" if they found any lawbreaking. Wall Street Journal sources claimed that the DOJ wouldn't ignore violations of other laws if discovered during the review.
The Department didn't name specific targets, but it's not hard to guess which tech giants are likely to come under the crosshairs. Amazon, Apple, Facebook, Google and Microsoft all exert tremendous influences on the internet, even if they don't always have a monopoly in a given area.
And those companies are likely to be nervous. They've already been asked to testify in front of Congress on competitive issues, and reports have circulated of regulators divvying oversight of Amazon and Google ahead of seemingly likely action. That's not including an imminent Facebook settlement with the FTC. There's a real chance they'll be asked to change their practices or even split into multiple companies.
You know these firms will put up stiff opposition, too. Apple has already denied that its App Store is a monopoly, and companies like Google have typically argued that you're not locked into using their web services the way you might be with software. They're determined to maintain the status quo, and they know the consequences of losing could be severe.