The hackers reportedly stole user names, email addresses and encrypted passwords. The company warned affected users and had their passwords reset.
The affected users seemed to be some of Twitter's earliest—those who set up accounts in 2006 or 2007, according to CNET.
"This attack was not the work of amateurs, and we do not believe it was an isolated incident," said Bob Lord, Twitter's director of information security, in a blog post. "The attackers were extremely sophisticated, and we believe other companies and organizations have also been recently similarly attacked."
"For that reason we felt that it was important to publicize this attack while we still gather information, and we are helping government and federal law enforcement in their effort to find and prosecute these attackers to make the Internet safer for all users," he added.
The hacking most likely occurred after an employee’s home or work computer was compromised through weaknesses in Java code, which is a popular computing language with well-known flaws, according to the Associated Press.
The attack comes as the Wall Street Journal revealed on Jan. 31 that it had been the victim of a major security breach by Chinese hackers, the AP reported.
The New York Times also announced it had been the victim of cyberspying, according to the Wall Street Journal.
"It is irresponsible to make such an allegation without solid proof and evidence," said Chinese Embassy spokesman Geng Shuang. "The Chinese government prohibits cyberattacks and has done what it can to combat such activities in accordance with Chinese laws."