The unclassified network of the Executive Office of the President—the administrative network of the White House—was breached by attackers thought to be working for the Russian government, according to multiple reports. The Washington Post reported that an investigation is ongoing, and White House officials are not saying what data, if any, was stolen from the computers on the network. “We are still assessing the activity of concern,” an unnamed White House official told the Post.

According to the Post’s anonymous sources, the breach was discovered in early October after a friendly foreign government alerted US officials. The network’s virtual private network access was shut down, and some staff members were told to change passwords. “We took immediate measures to evaluate and mitigate the activity,” the Post’s source at the White House said. “Unfortunately, some of that resulted in the disruption of regular services to users. But people were on it and are dealing with it.”

This isn’t the first time attackers, apparently sponsored by a foreign state, have targeted the White House’s network. In 2008 and 2012, Chinese hackers penetrated the White House’s network. On the first occasion, the attackers gained access to the White House’s e-mail server; in 2012, a phishing attack against White House staffers gave attackers access to the network, though officials said no sensitive data was exposed.