The end of the probe, which some FTC insiders characterized as a wrist slap by the Obama administration, irked staffers eager to delve into the search giant's Android mobile operating system, two sources with direct knowledge of the situation said.
Separately, the House Judiciary Committee in the US Congress announced an investigation into competition in digital markets.
Alphabet shares sank 6.12% or US$67.76, to US$1,038.74 on Monday's close, recording the biggest fall outside earnings since April 2011.
The Justice Department would have to file a lawsuit and convince judges that Google has undermined competition.
Google, which did not comment at the time, has previously said it has invested in China for years and plans to continue to do so, but that the company also was continuing to work with the US government on projects in healthcare, cybersecurity and other fields. Rep. David Cicilline (D-Rhode Island), who heads the panel's antitrust subcommittee, singled out Google LLC, Facebook Inc.
In the most extreme cases inquiries can lay the groundwork for breaking companies up, or forcing them to radically alter their practices.
Apple chief executive Tim Cook, speaking in an interview with U.S. television network CBS that aired on Tuesday, said the company controlled only a moderate share of the market.
"Their predatory power grabs demand strict & stiff investigation & antitrust action", the senator wrote on Twitter.
Just last Friday, the Department of Justice announced it was preparing an investigation of Google's business practices to see if the internet giant violated U.S. antitrust laws, Sputnik reported.
Facebook, Google, and Amazon stock all saw big declines on Monday.
Kevin Rippey, an analyst at Evercore ISI, said an investigation would add "to the near-term challenges" facing Google, but said it's unlikely an investigation would result in radical changes. The DOJ will examine Apple and Google, while the FTC will tackle Amazon and Facebook.
The Justice Department and FTC generally do not acknowledge preparations for any investigations. More competition from Amazon had a negative impact on the company's growth.
The European Union is investigating a complaint by streaming music provider Spotify Technology SA that Apple abuses its power over app downloads.
Trade commission investigators have been looking into Facebook's handling of privacy issues related to how it handles users' data for over a year. This March, the firm was hit with a US$1.7 billion fine for thwarting advertising rivals.