By Anne D'Innocenzio and Terry CollinsNEW YORK -- Social media giant Twitter is among the latest U.S. companies to report that it is among a growing list of victims of Internet security attacks, saying that hackers may have gained access to information on 250,000 of its more than 200 million active users. And now, The Washington Post ( WPO) is joining the chorus, revealing the discovery of a sophisticated cyberattack in 2011. Twitter said in a blog post on Friday it detected attempts to gain access to its user data earlier in the week. It shut down one attack moments after it was detected. But Twitter discovered that the attackers may have stolen user names, email addresses and encrypted passwords belonging to 250,000 users they describe as "a very small percentage of our users." The company reset the pilfered passwords and sent emails advising the affected users. The Twitter attack comes on the heels of recent hacks into the computer systems of U.S. companies, including The New York Times ( NYT) and The Wall Street Journal. Both newspapers reported this week that their computer systems had been infiltrated by China-based hackers, likely to monitor media coverage the Chinese government deems important. On Friday, The Washington Post disclosed in an article published on its Web site that it was also the target of a sophisticated cyberattack, which was discovered in 2011 and was first reported by an independent cybersecurity blog. Washington Post spokeswoman, Kris Coratti, didn't offer any details including the duration of the attack or the origins. But according to sources that the newspaper quoted, who it said spoke on condition of anonymity, the intruders gained access as early as 2008 or 2009. According to the sources, Chinese hackers are also suspected. Coratti couldn't be reached immediately for comment by The Associated Press. According to her comments made to the newspaper, the company worked with security company Mandiant to "detect, investigate and remediate the situation promptly at the end of 2011." China has been accused of mounting a widespread, aggressive cyberspying campaign for several years, trying to steal classified information and corporate secrets and to intimidate critics. The Chinese foreign ministry could not be reached for comment Saturday, but the Chinese government has said those accusations are baseless and that China itself is a victim of cyberattacks.