In what appears as the biggest cyber breach ever, Yahoo Inc recently said information of at least 500 million user accounts was stolen from its network in 2014 by what it believed was a state-sponsored actor.

Yahoo said data stolen may have included names, e-mail addresses, telephone numbers, dates of birth and encrypted passwords but that unprotected passwords, payment card data and bank account information did not appear to have been compromised.

“This is the biggest data breach ever,” said well-known cryptologist Bruce Schneier. He said it was too early to say what impact the breach might have on Yahoo and its users because many questions remain, including the identity of the state-sponsored hackers behind it.

Three US intelligence officials, who declined to be identified by name, said they believed the attack was state-sponsored because of its resemblance to previous hacks traced to Russian intelligence agencies or hackers acting at their direction.

Yahoo said it was working with law enforcement on the matter. The FBI said it was aware of the matter, and the US Secret Service was not immediately available for comment.