WASHINGTON (Reuters) - Some U.S. Department of Homeland Security employees in the Washington area and Philadelphia were unable to access some agency computer networks on Tuesday, according to three sources familiar with the matter.

A U.S. Customs and Border Protection agent applauds President Donald Trump's remarks at Homeland Security headquarters in Washington, U.S. January 25, 2017. REUTERS/Jonathan Ernst

It was not 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.

In a statement, a DHS official confirmed a network outage that temporarily affected four U.S. Citizenship and Immigration Services (USCIS) facilities in the Washington area due to an “expired DHS certificate.”

Reuters first reported the incident earlier Tuesday, which a source familiar with the matter said also affected a USCIS facility in Philadelphia.

Employees began experiencing problems logging into networks Tuesday morning due to a problem related to domain controllers, or servers that process authentication requests, which could not validate personal identity verification (PIV) cards used by federal workers and contractors to access certain information systems, according to the source.

Some employees were able to access systems through a virtual private network. It was not clear if other branches of DHS were affected.

The source characterized the issue as one stemming from relatively benign information technology missteps and a failure to ensure network redundancy. There was no evidence of foul play, the source said, adding that it appeared the domain controller credentials had expired on Monday when offices were closed for the federal Presidents Day holiday.

“We are working to track all device certificate issuance and expirations to ensure future lapses of service do not occur,” the DHS official said in the statement.

President Donald Trump vowed to make cyber security a priority during his administration, following an election marred by hacks of Democratic Party emails that U.S. intelligence agencies concluded were carried out by Russia in order to help Trump, a Republican, win. At a White House event last month he said he would “hold my Cabinet secretaries and agency heads accountable, totally accountable, for the cyber security of their organizations.”

Trump had planned to sign a cyber security executive order last month but it was put on hold to allow more time for review.