SAN FRANCISCO — Computer hackers swiped personal information from at least 500 million Yahoo accounts in what is believed to be the biggest digital break-in at an e-mail provider.

The massive security breakdown disclosed Thursday poses new headaches for beleaguered Yahoo CEO Marissa Mayer as she scrambles to close a $4.8 billion sale to Verizon.

The breach dates back to late 2014, raising questions about the checks and balances within Yahoo — a fallen Internet star that has been laying off staff and trimming expenses to counter a steep drop in revenue during the past eight years.

At the time of the break-in, Yahoo’s security team was led by Alex Stamos, a respected industry executive who left last year to take a similar job at Facebook.

Yahoo didn’t explain what took so long to uncover a heist that it blamed on a “state-sponsored actor” — parlance for a hacker working on behalf of a foreign government.

The Sunnyvale, California, company declined to explain how it reached its conclusions about the attack for security reasons, but said it is working with the FBI and other law enforcement. Yahoo began investigating a possible breach in July, around the time the tech site Motherboard reported that a hacker who uses the name “Peace” was trying to sell account information belonging to 200 million Yahoo users.

Yahoo didn’t find evidence of that reported hack, but additional digging later uncovered a far larger, allegedly state-sponsored attack.

The Yahoo theft represents the most accounts ever stolen from a single e-mail provider, according to computer security analyst Avivah Litan with the technology research firm Gartner Inc.

“It’s a shocking number,” Litan said. “This is a pretty big deal that is probably going to cost them tens of millions of dollars.”

Yahoo says it has more than 1 billion monthly users, although it hasn’t disclosed how many of those people have e-mail accounts. In July, 161 million people worldwide used Yahoo e-mail on personal computers, a 30 percent decline from the same time in 2014, according to the latest data from the research firm comScore.

The data stolen from Yahoo includes users’ names, e-mail addresses, telephone numbers, birth dates, scrambled passwords, and the security questions — and answers — used to verify an accountholder’s identity. The company said the attacker didn’t get any information about its users’ bank accounts or credit and debit cards.