The US Department of Homeland Security said on Thursday it is investigating a cyber breach affecting an information-sharing environment, as a media report identified the system as the Homeland Security Information Network.

The department said only that the incident involved an “unclassified legacy information sharing environment” and declined to provide additional details.

According to GovExec, citing unnamed sources, the breach affected the Homeland Security Information Network, a platform used to share sensitive but unclassified information with law enforcement agencies and other partners, and likely occurred between late May and early June.

Senator Mark Warner, the ranking Democrat on the Senate Intelligence Committee, said the information contained in the network was “highly sensitive” despite being unclassified and urged authorities to determine who was responsible for the breach and what information may have been compromised.