The Wall Street Journal took some of its computers housing news graphics offline after they were hacked, publisher Dow Jones & Co said.
People familiar with the matter told the Journal that the move was intended to isolate any possible attacks.
Workers were reviewing the systems to determine any signs of compromise, though they have not yet found any damage or tampering to the news graphics.
"At this point, we see no evidence of any impact to Dow Jones customers or customer data," a Journal spokeswoman said.
Dow Jones took down the systems after a hacker claimed on Twitter to have penetrated the wsj.com website, and offered to sell user information and the credentials needed to control the server.
The buyer could then "modify articles, add new content, insert malicious content in any page, add new users, delete users and so on," said IntelCrawler chief executive Andrew Komarov, who alerted the Journal to the hack.
Komarov said his firm has been monitoring the attacker, who goes by the nickname Worm (previously Revolver) and has founded Worm.in, a market for trading cyber vulnerabilities.