On Tuesday, one of the Senate’s fiercest tech critics, Sen. Ron Wyden (D-OR), called on regulators requesting that they investigate the ad-blocking industry for anti-competitive behavior.

For years now, some of the largest tech firms have paid ad-blocking companies like Eyeo, which owns Adblock Plus, to avoid the software’s restrictions and have their ads displayed on devices. In 2015, a report from the Financial Times showed that companies like Microsoft, Amazon, and Google were paying out ad blockers so that they could be added to a whitelist to avoid the software’s filters.

In a letter to the Federal Trade Commission, Wyden outlined this behavior and asked Chairman Joseph Simons to open an investigation into the entire ad-blocking industry as a response. Wyden argued that any company that accepts payment to be whitelisted should be “far more transparent” about the process with its users.

In the case of Adblock Plus, the company announced in 2016 that it would be accepting some ads that weren’t “intrusive or annoying.” The company whitelists these acceptable ads and allows them to run on devices, but Wyden argues that this behavior is “anti-competitive.

The FTC confirmed that it had received the letter.

“Hundreds of millions of consumers around the world have downloaded and installed software tools that purport to block online ads,” Wyden wrote in his letter. “In turn, the largest ad companies … have quietly paid millions of dollars to some of the largest ad blocking software companies in order to be able to continue to track and target consumers with ads.”

Over the past few years, the FTC has delegated more resources toward investigating competition problems in the tech space. Last February, the commission launched a task force of at least 17 attorneys who would focus on the technology space in light of new antitrust questions surrounding the market power of large tech companies like Google and Facebook.

Updated at 1/14/2020 at 11:21 a.m. ET: Added a note saying that the FTC had received Wyden’s letter.