A federal judge ruled Thursday that Google has illegally monopolized the online advertising technology market, handing the Department of Justice

In another major legal blow to Google, a federal judge Thursday ruled that the search giant held an illegal monopoly over some advertising technology.

A U.S. judge's ruling that Google has illegal monopolies in ad technology sets up the possibility of U.S. prosecutors seeking a breakup. Here's what the case involves and what Google owner Alphabet faces from here.