Six Democratic U.S. senators on Tuesday asked embattled Yahoo CEO Marissa Mayer for more information about the internet company's investigation into a hack that compromised data related to 500 million user accounts.

The lawmakers said they were "disturbed" the two-year-old intrusion was announced only last week.

"That means millions of Americans' data may have been compromised for two years," the senators wrote in a joint letter addressed to Mayer. "This is unacceptable."