The Guardian says its jobs site is now secure after suffering a "sophisticated and deliberate" hacking attack.
It emailed 500,000 people over the weekend warning them that their personal data may have been breached.
The Metropolitan Police's e-crime unit are investigating the hacking. The Guardian claims the police have asked it not to reveal how the hack was accomplished.
"The police remain anxious to keep information about the apparent theft to a minimum, in order not to compromise their investigations, but did agree with us that we could inform those users who may be affected. We have been assured by our provider that the system is now secure and, in line with the Information Commissioner’s guidance on data protection, we have identified and contacted, or attempted to contact, everyone who may be at risk," the Guardian said.
It is expected to be several days before the extent of the hacking of the site will be known. The precise number of people affected should become clear in the next few days. A Guardian technology director said: "We will have final numbers of real users and the type of data in next few days once we strip out duplicates, auto & false emails and so on. It's been reported that all user data has been compromised. which is simply not true."