The Department of Homeland Security said on Thursday it was investigating a breach in an unnamed information-sharing network.

In a statement, DHS said there had been a “recent cyber incident” involving an “unclassified legacy information sharing environment” but did not provide further details and did not respond to follow-up questions.

GovExec, which first reported the news, said the breach affected the Homeland Security Information Network, a platform used to share sensitive but unclassified data with partners, including foreign law enforcement, local authorities, and other organizations. GovExec, which cited two unnamed sources familiar with the matter, said the breach was believed to have occurred between late May and early June.

U.S. Senator Mark Warner, the top Democrat on the Senate Intelligence Committee, said the information carried in the network, “while not classified, is highly sensitive, and its exposure risks national security.”

He called on DHS and the Justice Department to “thoroughly investigate” who breached the network and what they compromised.

(Reporting by Raphael Satter in WashingtonEditing by Matthew Lewis)