(CNN) -- The company that was laying cable before an explosion last week at a popular Kansas City, Missouri, restaurant -- a blast that killed one person and injured at least 15 others -- did not have a permit for the excavation, a city official said Monday.
Those who fail to have such a permit could face a $500 fine and up to six months in jail, Assistant City Manager Patrick Klein said.
The contractor, Heartland Midwest, previously had worked in Kansas City after obtaining proper permits, according to Klein.
But he said the company did not do so last Tuesday, when one of their employees laying fiber optic cable hit a 2-inch gas line with an underground boring machine. The explosion occurred about an hour later, just after 6 p.m. CT (7 p.m. ET).
Authorities do not know what caused the leaking gas to ignite, according to Rob Hack, chief operating officer of Missouri Gas Energy.
"But all the damage points to ignition inside the restaurant," he said, referring to JJ's restaurant, a popular place near Country Club Plaza, an area filled with upscale shops and restaurants.
The contractor called 911 shortly before 5 p.m. to report the gas-line incident, according to Fire Chief Paul Berardi.
Mayor Sly James said city firefighters responded to a call about the gas odor a short time later but left the scene after being told by the utility company that everything was under control.
A Missouri Gas Energy employee showed up about 20 minutes after the initial call and, after some initial readings on gas-measuring devices, called for backup, Hack said. The utility's employees then went into businesses asking people to leave and brought in excavating equipment to try to vent the pipe, according to the Missouri Gas Energy official.
But while JJ's restaurant closed early due to an evident natural gas smell, hostess Deidre Estes and other employees weren't in a rush to leave, unaware of the danger they were in.
Then, "Boom! And everything was black," Estes told CNN affiliate KCTV. "... I saw the flames, and I was scared I was going to burn up. And then with all my might, I got this strength and lifted the rubble off."
The explosion had ripped off the restaurant's roof, sending flames a few stories high into the night sky. Afterward, debris -- including a destroyed backhoe belonging to the gas company -- was stacked 3- to 4-feet high where the restaurant once stood.
CNN's Steve Almasy contributed to this report.