More than 60 organizations — including unions, advocacy groups and small-business alliances — are urging the House Judiciary Committee to subpoena testimony from Mark Zuckerberg, Sundar Pichai, Tim Cook and Jeff Bezos.

In a pair of letters to the committee on Thursday, the groups argue that testimony under oath from the tech executives, alongside any relevant documents, is "critical" to the ongoing congressional investigation into whether Facebook, Google, Apple and Amazon have violated antitrust laws.

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"After a year of work on your bipartisan investigation, and with only six months remaining in the current legislative session, now is the time to act to ensure that the investigation can be brought to a successful conclusion and that any subsequent report can be written with full consideration given to all relevant materials," the groups wrote.

One of the letters was co-led by the American Economic Liberties Project, a progressive anti-monopoloy group, and the Internet Accountability Project, a conservative organization geared toward taking on Big Tech. The other includes sixty progressive organizations, including Demand Progress, the Electronic Frontier Foundation and several unions.

Over the past week, Facebook, Google and Amazon have all told the committee they are open to having their CEOs to testify — as long as other issues are resolved first, including the timing and format of the hearing, sources confirmed to Protocol. Sources said that representatives for Facebook and Google said their CEOs will testify only if the CEOs for their companies plus Amazon and Apple all appear jointly.

Apple has continued to hold out and has not yet confirmed that it is willing to offer testimony from Cook. Apple has argued that its business model is fundamentally different from the those of the other companies under investigation because it does not rely on user data to make money. But the committee has repeatedly questioned Apple's control over its App Store, and the company is currently facing two antitrust probes from the European Union.

Dave Segal, the executive director of digital rights organization Demand Progress, told Protocol that the letters are intended to ramp up pressure on House Judiciary Committee members to use their subpoena powers rather than allow the companies to dictate the terms of the testimony.

"As far as I'm concerned, as far as our coalition is concerned, these companies have not by any stretch actually agreed to testify voluntarily," Segal said. "There's a real tension between getting full compliance from the companies and being able to move forward expeditiously with the end of the investigation."

For more than a year, the Judiciary Committee has been investigating monopoly concerns around Facebook, Amazon, Google and Apple, questioning whether the companies have wielded their enormous market power to crush competitors and scoop up potential rivals. So far, lawmakers have threatened a subpoena only for Bezos.

The investigation is expected to wrap up over the next few months, and a source familiar with the probe told Protocol that lawmakers are hoping to secure testimony from the CEOs by the end of July.