The National Park Service says it is investigating what caused the elevator at the Washington Monument to malfunction two days after the building reopened.

Spokeswoman Carol Johnson said Wednesday about 60 people were on the observation level 500 feet up on May 14 and had to walk down nearly 900 steps.

The elevator stopped about 20 feet above the ground floor.

The monument reopened on May 12, nearly three years after an earthquake cracked and chipped it.

Johnson tweeted about 6:30 p.m. Wednesday that the elevator went out of service and about 50 visitors were being evacuated because of a thunderstorm.

She tweeted about 7 p.m. that the monument was open and the elevator running.