The House judiciary antitrust subcommittee sent letters to the four tech companies seeking communications and documents related to their market dominance.
"Today's document requests are an important milestone in this investigation as we work to obtain the information that our members need to make this determination," Rep. David Cicilline, D-R.I., said in a statement. "We expect stakeholders to use this opportunity to provide information to the committee to ensure that the internet is an engine for opportunity for everyone, not just a select few gatekeepers."
The letter to Google, for example, questions how the search engine's algorithm determines the ranking of search results and whether it downplays non-Google content or services that compete with Google.
Judiciary chairman Jerrold Nadler, D-N.Y., said there's growing evidence that a handful of corporations control the bulk of the online commerce and communication.
"It is increasingly difficult to use the Internet without relying on these services," Nadler said in the statement. "The documents requested will provide the committee with a better understanding of the degree to which these intermediaries enjoy market power, how they are using that market power, whether they are using their market power in ways that have harmed consumers and competition, and how Congress should respond."
The investigation has support from both sides of the aisle.
The committee wants to know whether anti-competitive behavior is occurring and whether current laws need to be improved, said Rep. Doug Collins, R-Ga.
Google already faces a separate Department of Justice investigation and a request from all 50 state attorneys general who want to look into antitrust violations.
Facebook is also part of an antitrust investigation conducted by the Federal Trade Commission.
Executives from the four companies testified to the antitrust subcommittee in July.
Cicilline accused them of being "evasive, incomplete or misleading" in response to basic questions about their market power.