The U.S. Justice Department has announced a major antitrust investigation into unnamed tech giants, while the House Judiciary Committee has begun an unprecedented antitrust probe into Google, Facebook, Amazon and Apple over their aggressive business practices, and promises "a top-to-bottom review of the market power held by giant tech platforms."

In addition, at least two 2020 presidential hopefuls have expressed support for breaking up some of technology's biggest players amid concerns they have become too powerful.

Experts say breakups are unlikely in the short term, and Rep. David Cicilline, the Rhode Island Democrat who leads the subcommittee pursuing the House investigation, called such measures a "last resort." But even without that, Facebook, Google, Amazon and Apple could face new restrictions on their power.

Google, Facebook, Amazon and Apple declined to comment on the investigations.

Here's a look at the cases that could be brought against them and what their defenses could be.

Read the full article on ABC News.