ST. LOUIS — A man with an assault rifle walked into a business early today and shot at least three people, police said. Their conditions were not immediately known.

The shooting occurred about 6:30 a.m. at the ABB Power plant on the city's north side, and two hours later, police believed the gunman was still in the building. Details about the victims were not immediately known.

Swiss-based ABB Group makes power transmission and industrial automation equipment. The company manufactures transformers at the St. Louis site, according to its Web site. It wasn't immediately known how many people work at the plant.

The St. Louis Post-Dispatch Web site, citing dispatcher reports, said some employees ran to the rooftop to escape the gunfire.

KMOV-TV reported nearly three dozen emergency vehicles circled the sprawling plant on a day made more chaotic by several inches of snowfall that have snarled traffic in the St. Louis region.

ABB has operations in roughly 100 countries, employing about 120,000 people.