Google, the search engine giant fined $5.1 billion over anticompetitive practices in Europe last year, tumbled in New York trading after reports that prosecutors are planning an antitrust investigation of the company in the U.S.

Shares in the Mountain View, Calif.-based company dropped 6.1% to $1,036.23 on Monday after the investigation was described in a Wall Street Journal article. The investigation marks the latest escalation in complaints that Silicon Valley standouts from Google to Facebook and Amazon have amassed monopoly powers, in part by competing with their own customers and advertisers.