There have been high-profile investigations by the EU into two other U.S. technology companies in the past 10 years. In both cases shares fell from the initiation to the resolution of the probes, although both rebounded in the two months following the end.

Intel Corp fell about 35 percent during an EU investigation between July 2007 and May 2009. After paying a fine, the Dow component rose nearly 10 percent over the subsequent two months.

The EU initiated three investigations into Microsoft Corp between 2008 and 2013. The first two were resolved without a fine, but over the period of the third investigation, which was resolved with a fine, the stock fell about 5 percent. It rose by roughly the same amount in the subsequent two months.

The EU's investigation into Google was first reported by media outlets on Tuesday, when the stock fell 1.6 percent. The company's shares closed up 0.2 percent at $541.04 on Wednesday.

(Reporting by Ryan Vlastelica; Editing by Christian Plumb and Andre Grenon)