Brewster Kahle, the founder and digital librarian, acknowledged the Distributed Denial of Service (DDoS) attack -- used to disrupt a website or server -- and said he was working to upgrade security.

The attack led to the "defacement of our website" and a breach of usernames, emails and passwords, Kahle wrote in a brief post on X, formerly Twitter, late Wednesday, without offering details.

In a new post on Thursday, Kahle said hackers had knocked down the Internet Archive's main site and its "Open Library," an open source catalogue of digitized books.

The Internet Archive is "being cautious and prioritizing keeping data safe at the expense of service availability," he wrote.

"Will share more as we know it," he added.

On Wednesday, users reported a pop-up message claiming the site had been hacked and the data of 31 million accounts breached.

"Have you ever felt like the Internet Archive runs on sticks and is constantly on the verge of suffering a catastrophic security breach?" the pop-up, apparently posted by hackers, said.

"It just happened. See 31 million of you on HIBP!"

HIBP refers to site called "Have I been Pwned," a site that allows users to check whether their emails and passwords have been leaked in data breaches.

In another post on X, HIBP confirmed that 31 million records from the Internet Archive had been stolen, including email addresses, screen names and passwords.

It was not immediately clear who was behind the operation.

Kahle did not respond to a request for comment about the scale and impact of the data breach.

The Internet Archive, a non-profit that operates a site called the Wayback Machine that saves snapshots of millions of web pages, was founded in 1996.

Like other archival sites, it is a crucial resource for fact-checkers, who use it to trace deleted web pages and ensure that the evidence cited in articles is permanently available to readers.

It can also be used to document changes made to online content over time and helps researchers and scholars find historical collections that exist in digital formats.