One month after a judge declared Google’s search engine an illegal monopoly, the tech giant faces another antitrust lawsuit that threatens to break up the company, this time over its advertising technology.

The Justice Department, joined by a coalition of states, and Google each made opening statements Monday to a federal judge in Alexandria, Virginia, who will decide whether Google holds a monopoly over online advertising technology.

The regulators contend that Google built, acquired and maintains a monopoly over the technology that matches online publishers to advertisers. Dominance over the software on both the buy side and the sell side of the transaction enables Google to keep as much as 36 cents on the dollar when it brokers sales between publishers and advertisers, the government contends.