The U.S. Postal Service has revealed it was hacked, an attack that has affected thousands of employees and millions of customers.

Ahough the company says an investigation is underway, the breach has called into question the security of the service, the last victim in a series of hacking attacks against American companies.

The postal service announced on Monday that hackers has managed to obtain the information of about 750,000 employees and retirees as well as the data of 2.9 million customers, according to CNN. The employees' stolen information includes birthdates, addresses, social security numbers, and employment codes used in the Postal Service's payroll systems.

"The Postal Service has recently learned of a cyber-security intrusion into some of our information systems. We began investigating this incident as soon as we learned of it, and we are cooperating with the investigation, which is ongoing. The investigation is being led by the Federal Bureau of Investigation and joined by other federal and postal investigatory agencies. The intrusion is limited in scope and all operations of the Postal Service are functioning normally," the USPS said in a statement.

According to USA Today, the USPS reported to Congress on Oct. 22 and Nov. 7 that it had been hacked. The briefings are classified but USPS spokeswoman Sue Brennan said the "issue is still under investigation."

Although the USPS website and cash registers were not compromised, the Customer Care Center may have been affected and anyone who contacted the center between Jan. 1 and Aug. 16 this year may be affected.

Rep. Elijah Cummings, D-Md., sent a letter to the USPS Postmaster General Patrick Donahoe on Monday asking for more information concerning the attack.

"It is an unfortunate fact of life these days that every organization connected to the Internet is a constant target for cyber intrusion activity," Donahoe said in a statement, according to the Associated Press. "The United States Postal Service is no different."