WASHINGTON — A major federal union said the cyber theft of employee information is more damaging than it first appeared, asserting that hackers stole personnel data and Social Security numbers for every federal employee.

The Obama administration had acknowledged that up to 4 million current and former employees are affected by the December cyber breach of Office of Personnel Management data, but it had been vague about exactly what was taken.

But J. David Cox, president of the American Federation of Government Employees, said in a letter Thursday to OPM director Katherine Archuleta that based on incomplete information OPM provided to the union, “We believe that the Central Personnel Data File was the targeted database, and that the hackers are now in possession of all personnel data for every federal employee, every federal retiree, and up to 1 million former federal employees.”

The OPM data file contains the records of non-military, non-intelligence executive branch employees, which covers most federal civilian employees but not, for example, members of Congress and their staffs.

The union believes the hackers stole military records and veterans’ status information, address, birth date, job and pay history, health insurance, life insurance, and pension information; and age, gender and race data, he said.