(CNN) -- It was happy hour at JJ's when the roof blew off the popular Kansas City, Missouri, restaurant.
"When we got to the scene, we had a fully involved restaurant that had patrons, probably several patrons, inside at the time of the incident," said Fire Chief Paul Berardi.
A natural gas explosion leveled the eatery about 6 p.m. CT Tuesday.
One woman, an employee of the restaurant, remains missing. A second person was missing before being found at an area hospital. At least 15 people were injured in the blast.
Search crews and cadaver dogs were unsuccessful in finding the missing person Tuesday, and crews with heavy equipment were awaiting daylight Wednesday to continue the search.
Debris was stacked 3 to 4 feet high where the restaurant once stood.
One man walked out from the rubble with only cuts and scratches, but he couldn't find colleagues who'd been standing next to him when the blast occurred.
"He was very worried about them," said Jill Chadwick, a spokeswoman for the University of Kansas Hospital. The patient gave her permission to relay his story but not divulge his name.
"He remembered the smell of gas, the explosion and the roof collapsing," Chadwick said. "He said, 'There was no door for me to walk through.' "
The blast had knocked out the front of the building, and he stepped through the gaping hole.
"Early indications are that a contractor doing underground work struck a natural gas line, but the investigation continues," according to the utility, Missouri Gas Energy.
The fire department suspects no "foul play at this time," Berardi said.
The gas leak started about an hour before the explosion.
People at JJ's and nearby residents smelled it and alerted authorities. Utility workers came out to inspect, fire department spokesman James Garrett said.
The fire chief confirmed that his department got a call about the gas leak about 50 minutes before the blast, but after consulting with the utility company, it was decided to leave the issue in their hands.
Jennifer Carter, who was in the restaurant, told CNN affiliate KSHB that a man who had a hand-held device came in before the blast and told her and 10 others to leave. Employees had already turned off appliances after smelling gas, she said.
Carter complied. She was just a few blocks away when she heard the explosion.
The gas sent flames a few stories high into the night sky. More than 100 firefighters responded, Berardi said.
Dr. Leonardo Lozada heard the explosion a block and a half away at St. Luke's Health Systems, where he is chief physician.
"It was pretty loud. I just heard it; it wasn't that traumatic," he said. Others told him they saw the roof blow off.
His hospital admitted two victims in critical condition along with others who suffered less severe injuries.
One man had burns to 40% of his body. KU Hospital also admitted a patient to its burn unit.
In all, hospitals reported three patients in critical condition and three in serious condition.
By 8 p.m., utility workers had turned off the flow of gas, Berardi said.
A crew of more than two dozen inspected nearby buildings to ensure no natural gas was trapped inside.
JJ's Restaurant was a popular place near Country Club Plaza, an opulent locale with shops and restaurants that offer much of what the heart desires with a proud price tag attached.
The menu at JJ's sported dishes with French names. Its wine list was a gourmet's dream and a penny pincher's nightmare. With more than 2,500 selections, a wine connoisseur could spend as much as $10,000 for the finest bottle in the house.
The restaurant's owner, James Frantze, was in Oklahoma at the time of the blast. A message on the restaurant's Facebook page made a simple request of patrons:
"Please keep our friends and families in your hearts and prayers."
-- CNN's Carma Hassan and Cristy Lenz contributed to this report.
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