Subpoenas were sent to the chief executives of the five largest tech companies on Wednesday as congressional Republicans moved to investigate what they assert is widespread corporate censorship of conservative voices.

Rep. Jim Jordan, R-Ohio, the chairman of the House Judiciary Committee, issued the subpoenas as the latest in a series of escalations by a party that has long promised to investigate Big Tech’s content moderation, especially when it came to COVID-19.

The letters were sent to Mark Zuckerberg of Meta; Sundar Pichai of Alphabet; Satya Nadella of Microsoft; Tim Cook of Apple; and Andy Jassy of Amazon.com.

And in them, Jordan outlined the committee’s objective to “understand how and to what extent the Executive Branch coerced and colluded with companies and their intermediaries to censor speech.”

Spokespeople for Microsoft and Meta said Wednesday that they have already begun producing documents. Requests for comment from Apple, Alphabet and Amazon was not immediately returned.

The committee asked the companies to produce documents and communications by March 23 that show any communication between them and the executive branch of the U.S. government relating to moderation, deletion, suppression or reduced circulation of content.

Notably missing from the list of companies subpoenaed is Twitter. The new owner, Tesla founder Elon Musk, has proven to be more sympathetic to conservatives than Twitter’s previous administration.

Just last week, three former Twitter executives appeared before the House Oversight and Accountability Committee to testify about the company’s decision to initially block a New York Post article in October 2020 about the contents of a laptop belonging to Hunter Biden.

The former employees conceded in that hearing that they made a mistake by blocking a story about the president’s son, from the social media platform in the run-up to the 2020 election, but adamantly denied GOP assertions they were pressured by Democrats and law enforcement to suppress the story.

In Wednesday’s letter, Jordan outlined how Musk’s decision last year to release a slew of company information to independent journalists “have exposed how Big Tech and the federal government have worked hand ways that undermine First Amendment principles.”

The documents and data, titled “the Twitter Files,” largely show internal debates among employees over the decision to temporarily censor links to the Hunter Biden story. The tweet threads lacked substantial evidence of a targeted influence campaign from Democrats or the FBI, which has denied any involvement in Twitter’s decision-making.

The hearing and subsequent subpoenas this month continue a yearslong trend of GOP leaders calling tech company leaders to testify about alleged political bias. Democrats, meanwhile, have pressed the companies on the spread of hate speech and misinformation on their platforms.