Personal information stolen from the home of a US government employee included data on 2.2 million military, officials said on Tuesday.
US veterans groups are now suing the government
It was previously thought the data only related to some 26 million veterans.
The Department of Veterans Affairs (VA) said as many as 1.1 million on active service, 430,000 National Guardsmen and 645,000 reservists may be affected.
There are concerns the data may be used to commit identity theft, but there is no evidence this has happened, it said.
The VA holds records for current members of the military as well as National Guard and reservists because they are eligible to receive certain VA benefits.
A coalition of veterans groups launched a lawsuit against the federal government on Tuesday, saying their privacy had been violated, reports say.
They are seeking $1,000 (£540) in damages for each affected veteran.
The data included names, Social Security numbers, dates of birth and details of some spouses.
The theft occurred last month when the home of the employee was burgled, although it was thought unlikely that the electronic data was the target of the burglary.
VA officials told Congress that he had been taking home data for three years.
The department previously said the employee was not authorised to take home the data and had been placed on administrative leave pending an investigation.