Questions are being raised about the possibility of a cyber-attack after Reuters reports several sources confirm U.S. Department of Homeland Security employees in the Washington area were unable to access some agency computer networks this morning.

It was not immediately clear how widespread the issue was or how significantly it affected daily functions at DHS, a large government agency whose responsibilities include immigration services, border security and cyber defense.

Employees began experiencing problems logging into networks at 5 a.m. ET on Tuesday due to a problem related to the personal identify verification (PIV) cards used by federal workers and contractors to access certain information systems, one source said.

At least four DHS buildings were affected, the source said, including locations used by U.S. Citizenship and Immigration Services.

Another source said the cards did not appear to be responsible. DHS did not immediately respond to requests for comment.