After the Federal Communications Commission said it was hit by an online attack over the weekend that prevented the agency from handling the public's comments on net neutrality, federal lawmakers are demanding to know exactly what happened.

In a letter to FCC Chairman Ajit Pai, Sens. Ron Wyden (D-Ore.) and Brian Schatz (D-Hawaii) are asking for specifics on the number of Americans who may have been prevented from submitting their feedback on Pai's deregulatory plan. The letter from Schatz — who chairs the Senate subcommittee that oversees the FCC — and Wyden, who has gained a reputation on cybersecurity and privacy, also asks whether the FCC has stress-tested its electronic filing system or uses any commercial software designed to block "denial-of-service" attacks. These attacks generally try to take down a website by spamming it with bogus traffic.

“A denial-of-service attack against the FCC’s website can prevent the public from being able to contribute to this process and have their voices heard,” the letter reads. “Any potentially hostile cyber activities that prevent Americans from being able to participate in a fair and transparent process must be treated as a serious issue.”

The FCC on Monday blamed the problems people experienced with its comments system on malicious attackers who had launched a denial-of-service attack against the agency's systems.

“These were deliberate attempts by external actors to bombard the FCC’s comment system with a high amount of traffic to our commercial cloud host,” said David Bray, the FCC's chief information officer, on Monday. “These actors were not attempting to file comments themselves; rather they made it difficult for legitimate commenters to access and file with the FCC.”

Fight for the Future, a group of Internet activists who support the FCC's current net neutrality rules, said Monday that it is “extremely skeptical” of the FCC's claim of an online attack and demanded that the agency release its website logs.

Tuesday's letter from Wyden and Schatz doesn't go as far. But it does ask for any evidence the FCC may have that outside actors are responsible for the outage.

The FCC's comment systems have become a battleground for grass-roots activists and broadband industry allies since Pai announced his intention last month to undo his Democratic predecessor's net neutrality rules. The rules required Internet providers to behave more like legacy telephone companies and ban the blocking or slowing of websites to consumers. And they also prohibit providers from charging websites a special fee to be displayed on consumers' screens — a tactic that consumer groups say would tilt the Internet playing field in favor of the providers' preferred or proprietary content companies.

Because of the ongoing war of words playing out in the FCC's comment system, the disruption to the system comes at a highly sensitive time. Comedian John Oliver on Sunday urged his viewers to submit comments to the FCC, but the agency's systems ground to a halt shortly after, because of the attack. The coincidental timing initially convinced some that Oliver was somehow responsible for causing an overwhelming surge of traffic to the site. But ultimately, the FCC said, it was the intentional denial-of-service attack, not a torrent of legitimate traffic, that caused the system to fail.

The FCC has until June 8 to respond to the senators' letter. The FCC said Tuesday that it has received the letter and is reviewing it.