The hackers likely were hoping the officials were conducting administration business on their private emails, according to lawmakers and security experts.

The government has acknowledged senior administration officials were targeted in the “phishing” attacks on hundreds of users of the email service. White House officials declined to discuss who was targeted.

The Obama administration reiterated Thursday that no official messages were compromised. But lawmakers and outside computer-security experts said recent White House history suggests administration officials sometimes use personal email to talk business, despite rules against doing so.

The Federal Bureau of Investigation and the Department of Homeland Security are working with Google to investigate. “These allegations are very serious,” Secretary of State Hillary Rodham Clinton said Thursday.