What happened

Shares of iRobot (IRBT 13.15%) had fallen by 8.4% as of 11:45 a.m. ET in Thursday trading, after Reuters reported that the European Union has launched a "full-scale" antitrust investigation of Amazon's (AMZN 3.43%) plan to acquire the maker of Roomba vacuum-cleaning robots.  

So what

Investors were presumably caught off guard by the announcement, which came just a week after the United Kingdom's Competition and Markets Authority gave its assent to the deal. Now, with the European Commission taking the opposite course, iRobot (and Amazon) investors have months of new uncertainty to look forward to.

Reuters notes that the E.U.'s antitrust regulator will conduct a "preliminary review" of the deal through July 6, which will be followed by a "four-month investigation." So in total, we're probably looking at five more months, minimum, before Amazon might be allowed to proceed with its purchase of iRobot.

Now what

Even if at the end of this process, the E.U. agrees with the U.K. that Amazon's purchase of iRobot does not raise competition concerns, investors still aren't 100% out of the woods. U.S. regulators are also looking into the deal, and may have concerns of their own. That being said, if everyone eventually signs off, then iRobot investors can expect to be paid $61 per share in cash -- which works out to a tidy 34% gain from Thursday's prices.

On the other hand, if the deal is ultimately called off, chances are that iRobot shares will revert toward the $50 level they were trading at just prior to Amazon's buyout offer -- or they could wind up even lower, near their current price of about $45, leaving investors worse off than they were before Amazon expressed its interest.

No wonder investors are bummed.