A new report from Northeastern University and the University of Massachusetts shows that U.S. wireless carriers are throttling online video traffic “24/7,” with AT&T and T-Mobile the biggest offenders. The claims come a little more than a year after the FCC’s repeal of net neutrality rules—that were designed to avoid such instances—went into effect.

Based on 650,000 tests from early 2018 to early 2019, the researchers found that AT&T throttled Netflix 70% of the time and Youtube 74% of the time. AT&T left Amazon’s Prime Video alone, however. T-Mobile U.S. throttled Amazon Prime 51% of the time but left Skype alone.

“They are doing it all the time, 24/7 and it’s not based on networks being overloaded,” David Choffnes, associate professor at Northeastern University, and one of the authors of the report, told Bloomberg.

[Read more: The Battle Over Net Neutrality]

The FCC repealed net neutrality rules in December 2017. At the time, critics warned that a repeal could lead to ISPs downgrading internet speeds that could favor their own content, a concern frequently raised in the AT&T-Time Warner merger. Attorneys General in 22 states have filed a protective petition for review against the FCC in the U.S. District Court of Appeals in the District of Columbia.

AT&T denied the claims.

“We don’t throttle, discriminate or degrade network performance based on content,” said AT&T spokesman Jim Greer in a statement. “We offer customers choice, including speeds and features to manage their data.”

FCC Commissioner Jessica Rosenworcel, a proponent of net neutrality, issued a stern warning about what could happen when the commission voted to repeal the rules in 2017.

“As a result of today’s misguided action, our broadband providers will get extraordinary new power from this agency,” Rosenworcel said at the time. “They will have the power to block websites, throttle services, and censor online content. They will have the right to discriminate and favor the internet traffic of those companies with whom they have pay-for-play arrangements and the right to consign all others to a slow and bumpy road.”