Story highlights Utility company said it appears contractor struck a gas line

Two people also missing, Kansas City fire department says

Witnesses say roof blew off restaurant

People in the area said they could smell gas well before explosion

Two people are missing and at least 14 people were injured when a gas line in Kansas City, Missouri, exploded and ignited a four-alarm fire, officials said Tuesday night.

The explosion apparently leveled JJ's restaurant, a popular spot in Country Club Plaza, an upscale shopping district and residential neighborhood that is a regional gathering place.

James Garrett, a spokesman for the Kansas City Fire Department, said three people were unaccounted for Tuesday night.

Missouri Gas Energy said: "Early indications are that a contractor doing underground work struck a natural gas line, but the investigation continues."

Mayor Sly James said the walls of the restaurant had collapsed.

JUST WATCHED Witness: Kansas City fire was 'huge' Replay More Videos ... MUST WATCH Witness: Kansas City fire was 'huge' 01:09

Dr. Leonardo Lozada, the chief physician at St Luke's Health Systems, said he heard the explosion at the hospital, which is a block and a half from the restaurant.

"Yes, it was pretty loud. I just heard it; it wasn't that traumatic," he said, adding that some people told him they saw the roof blow off.

Officials at three hospitals indicated at least 14 people were hospitalized, including three in critical condition and three in serious condition.

The number of injured had dropped from previous reports because there was "a lot of confusion with people coming into the emergency room." Lozada told CNN.

Two patients are in critical condition at St. Luke's Hospital, said Dr. Marc Larsen. One of the patients suffered extensive burns; the other facial trauma, he said. Most of the five other patients suffered minor lacerations or inhalation injuries, he said.

Lozardo said one man had burns to "40% percent of his body, mostly to his lower extremities."

Garrett said here were no fatalities at the scene, but two of those hurt had life-threatening injuries.

About 100 fire fighters fought the fire before it was put out, fire chief Paul Berardi said.

When the first fire trucks arrived the restaurant was fully involved with a few people inside, he said.

Witness says man told her to leave restaurant

One witness told CNN affiliate KSHB a man who had a hand-held device came into the restaurant before the blast and told her and 10 others in JJ's to leave.

Jennifer Carter told the station that restaurant workers already had turned off the appliances because people inside had smelled gas.

She said she left when told and was a few blocks away when she heard the explosion.

Garrett confirmed that workers from Missouri Gas Energy were "on the scene" before the explosion.

Residents who live near JJ's said they reported smelling gas to the utility company about an hour before the explosion, according to CNN affiliate KCTV.

The explosion shook doors and windows at nearby businesses and homes, the station reported.

James, who came to the scene from his office, said his primary focus was the victims.

"That's the main concern, making sure they come out OK," he said.

Garrett said gas lines have been shut off.

"People are still not allowed back into the plaza," he said. "I don't know when it will reopen -- hopefully tonight."