A pro-net neutrality group says the Federal Communications Commission (FCC) is hiding something in regard to the cyberattacks that brought the agency's website down this week.

“The public wants to know what the FCC is hiding,” said Evan Greer, campaign director of Fight for the Future. “They’ve been lying to us for weeks about net neutrality. It’s very difficult to accept their claims about [distributed denial of service] attacks when they have provided zero evidence to support them.”

The FCC's electronic filing system suffered a major slowdown Sunday night, shortly after HBO "Last Week Tonight" host John Oliver devoted a segment to net neutrality. The slowdown was thought to be the result of an influx of traffic after Oliver urged viewers to go the FCC’s site to file comments in support of net neutrality.

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But the FCC said later in the week it was the victim of a distributed denial of service (DDoS) attack that caused the system to go down Sunday night. DDoS attacks seek to bring down websites by bombarding them with phony traffic.

The agency also said this week that it was the victim of a DDoS attack in 2014, when the filing system went down after another Oliver segment on net neutrality. The FCC did not reveal the attack at the time.

Advocacy groups and at least one former FCC staffer expressed skepticism of both attacks.

“The official FCC position in 2014 was not that there was a DDoS attack following the John Oliver segment, because there was no clear evidence that such an attack occurred,” former top FCC Democratic staffer Gigi Sohn said in a statement on Thursday.

Groups like Fight For the Future and Color of Change, as well as Sens. Ron Wyden Ronald (Ron) Lee WydenGOP senator blocks Schumer resolution aimed at Biden probe as tensions run high Republican Senators raise concerns over Oracle-TikTok deal Hillicon Valley: TikTok, Oracle seek Trump's approval as clock winds down | Hackers arrested for allegedly defacing U.S. websites after death of Iranian general | 400K people register to vote on Snapchat MORE (D-Ore.) and Brian Schatz (D-Hawaii), called on the FCC to release proof of the attacks.

The agency did not say whether it would reveal evidence of either attack when questioned by The Hill on Tuesday.

FCC spokesperson Mark Wigfield told The Hill earlier in the week that the FCC had not revealed the 2014 attack out of concern for copycats. Wigfield said that the FCC’s system had the capacity to handle larger amounts of traffic than Oliver’s show was driving to the comment filing page.