WASHINGTON (AP) — 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 says 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.

Bob Vogel is superintendent of the National Mall and Memorial Parks. He said in an email statement Wednesday that the Park Service has asked the contractor for the elevator to consult specialists to determine the cause of the malfunction.