WASHINGTON  -- More than a million victims of a massive hack of U.S. government computer files have still not been officially notified that their data was compromised and that they are eligible for free credit-monitoring protection, officials said on Friday.

The government this week finished sending notifications through the Postal Service to 21.5 million people affected by the breaches, said the Office of Personnel Management (OPM), the federal hiring agency that was hacked.

The intrusions, linked to China, began in May 2014 and were not discovered and announced publicly until a year later.

The postal notifications should be received by the middle of next week, but about 7.0 percent of those hacked, or roughly 1.5 million people, could not receive notification letters because their addresses have changed or are not on file, OPM said.

The hack exposed names, addresses, Social Security numbers and other sensitive information for current and former federal employees and contractors, as well as applicants for federal jobs and individuals listed on background check forms.

In an interview on Friday, an OPM spokesman said it would resend postal notices to updated or changed addresses and rely on a "media campaign" to tell people they can check online to see if their information was hacked.

“We’re going to clean up that 7.0 percent and get as close to 100 percent as possible," OPM spokesman Sam Schumach said, calling 93-percent notification "a really high percentage."

OPM will not rely on email notifications to close the gap. Victims of a smaller, related OPM hack were notified by email and given instructions about what to do, but some experts said the emails unfortunately resembled a phishing scam.

"It's just not as secure," Clifton Triplett, OPM’s newly appointed cyber adviser, told Reuters on Friday.

The government awarded technology firm Advanced Onion a $1.8 million contract to help locate and notify those affected by the data heist. More than $130 million was awarded to Identity Theft Guard Solutions to provide victims credit and identity-theft insurance for three years.

Cybersecurity researchers have said there is no indication that information from the hack has appeared for sale on online black markets and that this suggest the Chinese government, not criminals, stole the data trove.

(Editing by Kevin Drawbaugh)