St. Louis ? A fire broke out Thursday on the grounds of the Gateway Arch, prompting officials to evacuate thousands of visitors from the famed St. Louis landmark.

Thick black smoke could be seen hovering over downtown for miles. No injuries were reported.

Officials at the National Park Service said visitors at the Museum of Westward Expansion beneath the Arch were evacuated, as were those who travel by tram to the top of the monument. A total of about 8,000 people were removed from the Arch and its grounds, superintendent Gary Easton said.

The fire apparently started at an electrical transformer about 50 feet from the Arch. The cause was not immediately known.

Easton said the transformer was one of three that provided power to the Arch. The museum and the Arch could be shut down up to three days for repairs, he said.

The FBI was investigating because national monuments such as the Arch have been cited as potential terrorism targets. But Easton said there was no reason to believe the fire was the result of anything other than a malfunction.