The head of the Office of Personnel Management refused on Wednesday to estimate how many millions of Americans were affected by recent hack attacks on her agency's computers, but rejected as premature reports 18 million were compromised.
OPM Director Katherine Archuleta conceded the number of people whose data was hacked could increase from initial estimates.
OPM said this month it was victim of a cyber attack involving personnel data of 4.2 million current and former federal employees. Another attack targeted information of millions more Americans who applied for security clearances. Some media reports said 18 million were affected.
"It is my understanding that the 18 million refers to a preliminary, unverified and approximate number of unique social security numbers in the background investigations data," Archuleta said at a House of Representatives hearing.
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"It is a number that I am not comfortable with at this time."
In her second appearance in as many weeks before the House Oversight and Government Reform Committee, Archuleta defended her agency's response to the hacking incidents, the most recent of which was detected in April. It was the second of three Capitol Hill hearings this week on the OPM breaches.
