An explosion leveled a small commercial building Tuesday, killing three workers and critically injuring a fourth.

The blast in this Minneapolis suburb caved the single-story building into its empty basement, sheriff's Capt. Robert Aldrich (search) said. The site was strewn with concrete chunks, cinderblocks and shreds of office furniture.

A gas leak was the most likely cause of the explosion, but nothing was immediately ruled out, Aldrich said.

The building housed a real estate business and bank offices.

Bob Smith, 60, was hospitalized in critical condition with burns. The three dead workers were women, officials said. Their names were not immediately released.

The body of the third woman was found after a daylong search with two backhoes and a crane.

Ramsey is a town of some 12,500 people about 25 miles northwest of Minneapolis.