St. Louis ? Less than half an hour after park officials declared the Gateway Arch safe and partly operational Sunday afternoon, the lights inside went out, the tram shut down and people started panicking – again.

This time, the power was restored in about 15 minutes, unlike the three-hour ordeal visitors had suffered through the night before.

Arch Superintendent Peggy O’Dell said workers investigating damage on the disabled south tram of the Arch triggered the brief outage Sunday on the north side.

On Saturday, a cable that broke in the south leg of the Arch may have toppled onto an electrified railing around 8:45 p.m. CDT and shorted the electrical circuits. Power was restored to the north tram around 11 p.m. Saturday – but the south tram remains out.

About 260 people, including many young children and elderly visitors, were trapped in darkness and heat without access to food, water or restrooms. No one was seriously injured, although it took firefighters and park staffers up to three hours to clear everyone from the monument.

Park officials said the trams, which travel up and down the Arch and consist of eight cars or pods, were never in danger of falling because they are secured by nine cables and need only two or three of those to stay connected.

The 630-foot-tall structure is operated by the National Park Service and opened in 1966. It is one of the region’s most popular attractions.