Though the intruders failed to gain access to passwords or other personal data, Washington Post said that some of the users may receive junk e-mail or spam. The company also said that the website which allows users to search job listings and post resumes, is under control and data is secure.
"We are taking this incident very seriously. We sincerely apologize for this inconvenience," Washington Post said in a statement.
The Post spokeswoman Kris Coratti said they will reset the security system and will carry on the investigations after discussing the issue with a law enforcement authority.
"We wanted to make sure that we had a complete understanding as to what had happened and the potential consequences so that we could provide our customers with information that was as accurate and helpful as possible," Coratti said.
Earlier, hackers had intruded into Fox News' Twitter account and tweeted false news on the United States President Barack Obama's assassination.