The official tally of those affected by the breach of Office of Personnel Management networks is expected to rise this week.

OPM is preparing to inform millions of current, former and potential federal employees that their information might have been exposed in the breach of background investigation data unveiled on June 8, commonly referred to as the second OPM hack.

Officials confirmed the agency is planning to send out notices to all those potentially affected by the breach on a rolling basis scheduled to begin this week.

With that, the agency likely will also finally put a figure on the number of those affected, projected to be as high as 18 million by some accounts.

Information on more than 4 million federal employees was exposed in the initial breach of personnel data, however this second intrusion covers a much larger pool, potentially affecting anyone who ever applied for security clearance, including some military personnel.

OPM Data Breach: What You Need to Know

The kind of information compromised in the second breach included Standard Form 86, an intrusive form that asks applicants to disclose highly personal information, such as psychological exams and details about family and close friends.

"Thanks to the tireless efforts of my team at OPM and our interagency partners, we … have made progress in the investigation into the attacks on OPM's background information systems," OPM Director Katherine Archuleta said in a July 4 blog post. "I share your anger that adversaries targeted OPM data. And I remain committed to improving the IT issues that have plagued OPM for decades."

The agency also pulled its e-QIP online submission system offline last week to fix a critical security vulnerability. Applicants for security clearances will have to file hardcopy forms for the next four to six weeks while the application is down.