Lauber and chief prosecutor Carlo Bulletti held a press conference in Bern on Thursday to reveal that the employee, a man who worked for the Swiss intelligence service, wanted to sell the data to foreign countries.
The case involved a serious matter of economic sabotage, Bulletti said.
The prosecutors did not specify the nature of the stolen data.
But Bulletti invited speculation by adding, “I can all the same imagine that this information had a considerable market value.”
The intelligence service informed the attorney general’s department on May 25 that it suspected being the target of data theft, he told reporters.
The department immediately launched an investigation with the help of federal judicial police.
The IT worker was arrested and placed in preventive detention until July 5.
He was someone who had the authority to handle the sensitive data but who appeared to be suffering from psychological problems, the prosecutors said.
The attorney-general’s department had not planned to make public information about the espionage.
However, the federal department of defence on Wednesday issued a news release noting that a criminal complaint had been lodged against an employee of the intelligence service in connection with the stolen data.
The defence department said the theft was detected thanks to “security controls” that allowed the intelligence service to respond quickly.
All the stolen data was retrieved and its transfer to third parties was prevented, the department said.
Lauber said the attorney-general’s department wanted to shed light on the affair to “avoid speculation against Switzerland”.
A court case is expected to follow the criminal investigation.