The U.S. Postal Service said Monday that it was hacked and employee information, including Social Security numbers, may have been stolen.
The Postal Service did not say who was suspected of breaking into its systems. It called the breach "limited in scope."
The FBI and other federal agencies are investigating.
The cyberattack did not affect the systems that handle customer transactions, the agency said.
"There is no evidence that any customer credit card information from retail or online purchases such as Click-N-Ship, the Postal Store, PostalOne!, change of address or other services was compromised," the Postal Service said in a statement.
However, hackers did gain access to the Postal Service's call center data. Customers who contacted the center by phone or email from Jan. 1, 2014, to Aug. 16, 2014, may have had their names, addresses, phone numbers and other data stolen, if they provided such information.
The Washington Post reported that Chinese government hackers are top suspects. The Postal Service said the source of the attack remains under investigation and that it couldn't comment further.
An FBI spokesman said he could not comment on an ongoing investigation.