Six U.S. senators have called Yahoo’s massive data breach “unacceptable,” and they’re demanding that the company provide more details about the incident.

In a letter addressed to Yahoo’s CEO, the lawmakers said they were particularly “disturbed” that the breach occurred in 2014, but that Yahoo publicized it only last week.

“That means millions of Americans’ data may have been compromised for two years,” the letter said. “This is unacceptable.”

The hacking incident, which Yahoo said it only learned recently, affects at least 500 million user accounts, making it perhaps the largest known data breach in history. Account information, including email addresses, telephone numbers and hashed passwords, may have been stolen.

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