WASHINGTON – A congressional antitrust investigation into Amazon, Apple, Facebook and Google entered a new phase Friday, after lawmakers called on each of the tech giants to turn over a trove of sensitive documents, including top executives’ private communications.

The requests sent by Democrats and Republicans on the House Judiciary Committee ask the companies to share detailed information about their internal operations, including financial data about their products and services, private discussions about potential merger targets and records related to “any prior investigation” they have faced on competition grounds.

The documents could shed light on whether the companies’ dominance of search, advertising, e-commerce and other digital markets is rooted in anti-competitive practices, such as gobbling up or squashing rivals, and the extent to which their leaders participated in, or had been personally aware of, any wrongdoing. The lawmakers’ letters are not official legal demands, though the panel does have key powers to compel the four tech giants to turn over records or appear at hearings if necessary.

Separately, Rep. David Cicilline, D-R.I., the chairman of the panel’s antitrust subcommittee, on Friday fired off 427 detailed follow-up questions to Apple, Amazon, Facebook and Google that touch on many issues raised when the companies’ executives testified at an antitrust hearing in July.

The queries illustrate the exhaustive, wide-ranging nature of the congressional review: It asks about Amazon’s data-collection efforts about sellers on its platform, Apple’s policies around third-party apps, Facebook’s tools to monitor competing services and seeks more information on practically every element of Google’s sprawling empire in search, smartphones, advertising, mapping, online video and more.

“The open Internet has delivered enormous benefits to Americans, including a surge of economic opportunity, massive investment, and new pathways for education online,” Rep. Jerry Nadler, D-N.Y., the chairman of the Judiciary Committee, said in a statement. “But there is growing evidence that a handful of corporations have come to capture an outsized share of online commerce and communications.”

His Republican counterpart, Rep. Doug Collins of Georgia, added: “This information is key in helping determine whether anticompetitive behavior is occurring, whether our antitrust enforcement agencies should investigate specific issues and whether or not our antitrust laws need improvement to better promote competition in the digital markets.”

Apple, Amazon and Facebook did not immediately respond to requests for comment. Google declined, pointing to prior statements by Kent Walker, its senior vice president of global affairs. “We have always worked constructively with regulators and we will continue to do so,” he said earlier this month.

The demands from House lawmakers add to the antitrust pressure on Silicon Valley, as regulators around the country begin to question whether tech giants threaten innovation, smother new start-ups and result in higher prices or worse online services for consumers.

On Monday, attorneys general from nearly every U.S. state and territory announced an investigation into Google, focusing initially on the online-advertising machine that powers its search engine and bottom line. The company also is facing a probe by the Department of Justice, part of a broad look at the competition issues posed by the tech industry. State and federal authorities each have issued Google formal legal demands for records related to their investigations.

A week earlier, 11 state attorneys general said they’d open a competition-minded inquiry of Facebook, including the way it taps billions of users’ personal data. In Washington, the Federal Trade Commission also is exploring the company, focused on its prior acquisitions, an outgrowth of the FTC’s wide-ranging review of the industry.

House lawmakers launched their investigation in June: Cicilline, the Rhode Island Democrat who chairs the House Judiciary antitrust subcommittee, at the time described the internet as “broken,” before highlighting the “reluctance in the early days of the Internet to interfere” among government. A month later, the Judiciary Committee grilled officials from Apple, Amazon, Facebook and Google at a hearing, where Cicilline pointed to a web that had become “increasingly concentrated, less open, and growingly hostile to innovation and entrepreneurship.”

On Friday, Cicilline said the committee’s requests for information marked an “important milestone in this investigation.”

“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,” he said.

With Google, lawmakers have asked Google’s parent company, Alphabet, to turn over internal emails and other records related to its prior mergers, such as its purchase of YouTube, along with documents it’s turned over to other governments in connection with previous antitrust investigations. Google has faced multiple such probes in recent years, especially in the European Union, which has fined the company more than $9 billion for violating competition laws.

With Facebook, meanwhile, lawmakers asked the social-networking giant to turn over records in which CEO Mark Zuckerberg might have talked about corporate rivals, including the since-shuttered app Vine and two services Facebook later acquired, Instagram and WhatsApp. Investigators also are seeking additional documents related to Facebook’s relationships with third-party app developers.

Lawmakers asked Amazon to provide extensive records related to the way in which it prices and displays its products alongside those sold by rival sellers, along with additional records related to its many acquisitions, including the security firm Ring and the grocery chain Whole Foods. (Amazon founder Jeff Bezos owns The Washington Post.) With it, they posed more than 150 questions to the tech giant related to the data it collects about sellers on its platform and users of its Alexa and Echo products.

And in taking aim at Apple, Democrats and Republicans sought more information about the way it manages its App Store, including its controversial decision earlier this year to remove parental-control tools. Lawmakers also asked Apple about reports that it copies popular apps’ features, and requested details on the inner-workings of its algorithm that determines where apps are ranked in search results on the store.