WASHINGTON (AP) — Officials are investigating after a prominent part of Washington’s skyline went dark.

The National Park Service tweeted Tuesday night that the Washington Monument’s lights went out around 7 p.m.

Park service spokesman Mike Litterst said at about 9:15 a.m. Wednesday that the lights were on when electricians arrived in the morning, “indicating the lighting system’s clock is likely out of sync.” He said that park service staff are working to correct the problem and that the lights are expected to be on Wednesday evening as normal.

The monument has been closed to visitors since August because of ongoing problems with its elevator, which carries visitors to an observation deck near the top of the 555-foot tall monument. It usually draws 600,000 visitors a year.

In December, officials announced a $2 million to $3 million project to modernize the elevator. The monument is expected to reopen to visitors in 2019.

WTOP’s Rick Massimo contributed to this report.