OMAHA, Neb. (AP) — Investigators are working to determine what caused an explosion and fire that caused extensive damage to a historic downtown Omaha building that housed a restaurant and apartments.

Several witnesses say they smelled natural gas at M’s Pub shortly before the explosion and subsequent fire that started Saturday afternoon.

One man was seriously injured, yet no one was killed in the fire.

Omaha Fire Chief Bernie Kanger said Sunday morning that the fire was essentially out, but crews were still monitoring hot spots for flare ups.

The outside of the four-story brick building was encased in icicles from all the water firefighters sprayed in this weekend’s frigid weather.

Kanger says investigators haven’t determined the cause of the fire or whether the structure is salvageable.