The spate of mergers and acquisitions that have dominated the online advertising industry of late has gotten a lot of press, but until now only Google's DoubleClick purchase has sparked a Federal Trade Commission investigation. Now, The Wall Street Journal reports that the FTC plans to put Yahoo! and Microsoft under its microscope.

Under the Hart-Scott-Rodino antitrust act, deals like Microsoft's $6 billion aQuantive buy and Yahoo's $680 million purchase of 80 percent of Right Media are subject to scrutiny of federal regulators. But so far, the FTC has asked only Google to provide so-called second requests, which indicate a higher level of scrutiny.

Officials from Microsoft and Yahoo said they were cooperating fully with the FTC.