WASHINGTON (AP) — The Washington Monument will be closed indefinitely because of ongoing problems with its elevator, the latest in a series of woes for one of the city’s most enduring tourist attractions.

The National Park Service announced Monday that the monument, which draws 600,000 visitors a year, will remain closed until its elevator control system can be modernized, a process expected to take up to nine months once work begins. A start date for the $3 million project has not been determined.

The lone elevator that takes visitors to the top of the 555-foot obelisk has broken down frequently over the past two years, roughly since the monument reopened after being damaged in an earthquake.

Park service officials have said they don’t believe that the 2011 earthquake caused the elevator problems. But they don’t know exactly what’s wrong with it. Despite a month-long inspection, “we have not been able to determine the cause of the ongoing reliability issues,” the park service statement said.