Representatives for Experian, Equifax, and TransUnion said they are co-operating with authorities investigating the mysterious website that has posted private financial data of stars such as Jay-Z, Tiger Woods, the vice-president, and first lady Michelle Obama.
The companies said they were also working to minimise the financial damage to the victims. Experian spokesman Gerry Tschopp said the company had frozen credit files of those whose information has been posted online.
The FBI and Secret Service are investigating the site, which remains active.
Equifax spokesman Tim Klein said that an initial investigation showed the hackers exploited a website designed to give consumers a free credit report. The three companies state that the hackers used a wealth of personal information on their victims to impersonate them and generate the credit reports.
Although vice-president Joe Biden and former secretary of state Hillary Clinton are included on the site, their credit reports have not been posted.
The site includes social security numbers, credit reports, addresses, and phone numbers on most of its victims.
It bears an internet suffix originally assigned to the former Soviet Union, and many of the pages feature unflattering pictures or taunting messages of the person featured. Others whose information is posted include Britney Spears, US attorney general Eric Holder, Donald Trump, Sarah Palin, and Arnold Schwarzenegger.
A counter on the website indicated that it had received more than 450,000 views since its existence was revealed this week.
Social security numbers posted on Jay-Z, Mel Gibson, and others matched records in public databases.
Online security expert Marc Maiffret said sensitive information can often be gleaned from a single database, but the varied nature of the people targeted made the site’s motives less clear.
The site’s so-called secret files claim to reveal everything from how much Kim Kardashian pays for her car lease to Ashton Kutcher’s American Express bill and even Paris Hilton’s credit score.
White House press secretary Jay Carney said he had “no assessments to offer” on the situation and referred questions to the Secret Service, which would not provide further details.
The site grew from 11 names to 18 in the first 24 hours since it became public, with its operator adding additional features to count the number of visitors and a link to a Twitter account.
It offers no explanation about why the targets were selected or how the information was obtained. The Twitter account includes an anti-police message in Russian.
President Barack Obama told ABC News that he was aware of the investigation.
“We should not be surprised that if we’ve got hackers that want to dig in and have a lot of resources, that they can access this information,” he said.
“Again, not sure how accurate but... you’ve got websites out there that tell people’s credit card info. That’s how sophisticated they are.”