The U.S. State Department has shut down its unclassified email system after a reported cyber attack.

The State Department disabled the system on Friday over concerns about suspicious network activity, although its classified systems remained unaffected, according to Agence France Presse.

Initially, government officials said the shutdown was mere routine maintenance to its main unclassified network, affecting only email and public website access. But a senior official told a different story, according to AFP, noting that the department had detected “activity of concern” in parts of the system that handled non-classified emails. The weekend maintenance was altered to include security improvements to handle the breach, according to the report.

The official added that none of the classified systems were ever in danger.

It is the latest in a string of cyber attacks on government agencies. The revelation comes just a week after the U.S. Postal Service reported hackers stole sensitive personal info in a major data breach. A USPS spokesman said as many as 800,000 people who were paid by the agency, such as employees and private contractors, had their personal info compromised. USPS is working with the FBI and other law enforcement to determine the culprits.

The White House itself was the victim of a cyber attack on its unclassified computer network last month.

White House users were disconnected from the network temporarily while officials dealt with the breach, although the computers and systems themselves remained undamaged. A Washington Post report suggests that the Russians were behind that attack.