The Federal Communications Commission (FCC) has launched the regulatory review for T-Mobile’s proposed $26 billion acquisition of Sprint.

The FCC on Friday opened a docket for the merger and issued a protective order allowing the companies to shield sensitive documents submitted to the agency from public view.

A spokeswoman for FCC Chairman Ajit Pai did not immediately respond when asked for comment, but Pai wrote to members of Congress last week promising to “thoroughly review” whether the deal is in the public interest.

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Democratic lawmakers have already expressed concerns about the merger, which would reduce the number of major national wireless carriers from four to three. Critics say that eliminating a competitor from an industry that’s so concentrated would hurt consumers by reducing their options in the market and raising prices.

T-Mobile and Sprint have argued that the combination is necessary in order for them to compete with Verizon and AT&T in new technologies like building 5th-generation wireless networks.