The Justice Department’s “Antitrust Division Manual” allows a case to be developed through a grand jury if there is evidence showing anticompetitive behavior by a company. Information gathered through a civil inquiry that they sought to destroy competition to gain a monopoly could become the basis for opening a criminal investigation.

The primary law used to police monopolies is Section 1 of the Sherman Act, which prohibits “every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce.” The statute does not define what a monopoly is, but the F.T.C. has noted that courts view a firm with less than 50 percent of the sales of a particular product or service as one without sufficient market power to constitute a monopoly.

For Google and Apple, which are likely to be investigated by the Justice Department’s antitrust division, the key issue will be determining what market, if any, they have enough control over to be considered a monopoly. In cloud computing, for example, Google is far behind Microsoft and Amazon. But in internet searches, Google could well be considered to have something close to monopoly power, especially when its very name has become a verb for searching for information.

A second avenue the Justice Department could pursue is under Section 2 of the Sherman Act, which prohibits an “attempt to monopolize” trade or commerce. This could be a more amenable path to establishing an antitrust violation because it does not require proving that there is an actual monopoly, only that steps were taken to acquire monopoly power. A document issued by the Justice Department called “An Antitrust Primer for Federal Law Enforcement Personnel” notes that Section 2 violations “are generally not prosecuted criminally,” but if there is enough evidence to show attempted monopolization then there is nothing to prevent prosecutors from pursuing a criminal case.

But to do so, the Supreme Court explained, in the 1993 case Spectrum Sports v. McQuillan, that the government must prove three things: “(1) that the defendant has engaged in predatory or anticompetitive conduct with (2) a specific intent to monopolize and (3) a dangerous probability of achieving monopoly power.” The specific intent to monopolize does not mean that one seeks monopoly power by competing vigorously, but that it is an “intent to destroy competition or build monopoly.” That means a company’s efforts to drive others out of business could be proof that it has the intent to become a monopoly.