Yesterday's installment of Today's Paperswas upset that no newspaper explained why the FTC was prosecuting Intel when the Department of Justice is prosecuting Microsoft. Well, please don't accuse Explainer of immodesty, but one reason the New York Times and Washington Post might have ducked the question is that it was settled right herein this space way back in June.
The answer is that the FTC and Department of Justice have so-called "concurrent jurisdiction" over antitrust, meaning that both are supposed to chase antitrust violators. There is a joint committee charged with divvying up cases to avoid duplicate investigations. The committee assigns merger reviews to the agency best acquainted with that industry; a non-merger investigation generally goes to the first agency that proposes it. Sometimes--as in the Microsoft case--one agency (the FTC) will begin an investigation but ultimately drop it, and the other (DOJ) will pick it up.
That's not all. States may write and enforce their own antitrust law even, says the Supreme Court, if it contradicts federal law. (This is an unusual arrangement since the courts often hold that aspects of economic policy are "pre-empted" by the federal government under the Commerce Clause of the Constitution.) State attorneys general may also sue in federal courts using federal law. Often state and federal agencies investigate and prosecute together, as in the Microsoft case. And victimized individuals or firms may sue for damages, under state and/or federal law. Federal law and some state laws award triple damages plus legal costs if you win.