WASHINGTON (NewsNation Now) — You might have seen “White House Black Out” trending on Twitter Monday morning, accompanied by photos of the Washington Monument without its usual illuminating light. This left some social media users wondering what was going on at the nation’s Capitol.

On Sunday evening, the official Twitter for D.C.’s National Mall and Memorial Parks tweeted that “the exterior lights of the Washington Monument [were] currently out” and that an investigation was ongoing.

NewsNation reached out to Mike Litterst, spokesperson at the National Mall and Memorial Parks, who stated the outage was caused by a timeclock failure that controls turning the lights off and on.

“The lighting outage at the Washington Monument was caused by the failure of the timeclock that turns the lights on each evening and off in the morning. While the faulty timeclock is being replaced, the lights will be operated manually to ensure the monument is lit each night. We are not aware that the White House lights were out last night.”

Statement from Mike Litterst, Chief communications at National mall and memorial parks

While some online users reported the White House going dark as well Sunday night, NewsNation’s Washington bureau confirmed the White House lights go out around 11 p.m. ET nightly, and it is not unusual.

The official White House Twitter account did not comment on the trending topic.