The blast happened around 6 p.m. Tuesday, when the dinner crowd would have been filing into JJ's and the many other restaurants in the upscale Country Club Plaza shopping and dining district.
The restaurant was a fixture on the city's culinary scene for more than 27 years. Locals knew it as a prime after-work stop, though it won a broader reputation after receiving consistently high ratings from contributors to Zagat's restaurant guides, both for its food and its long wine list.
Firefighters received a call about 5:15 p.m. that a construction worker had hit a gas line near the restaurant, and they conferred with employees of Missouri Gas Energy, which supplies the area.
It wasn't clear Wednesday how hard firefighters or utility officials worked to evacuate the restaurant after gas was first noticed. Fire Chief Paul Berardi said the fire department deferred to the utility since it would have more expertise in assessing the seriousness of the situation.
Richard Kuprewicz, a pipeline safety consultant in Redmond, Wash., said federal law holds the utility responsible for deciding whether to evacuate, but assessing the risk isn't always easy.
Sometimes it's difficult to determine how much gas has been built up. And even highly trained people can underestimate the danger.
"I've seen people who work for gas companies and have gas sniffers, and their bodies are found in buildings," Kuprewicz said. "There is some art and some experience and some training in this stuff."
The fire chief said the precise cause of the gas leak is still under investigation.
A construction project had been going on across a narrow, one-way street from JJ's for seven years. But it was not clear Wednesday whether that work was connected to the contractor that MGE said had been underground.
MGE planned to issue a statement later Wednesday.
The Missouri Public Service Commission, which oversees utilities, launched an investigation into the blast, dispatching five employees to the site.
Commission Chairman Kevin Gunn said preliminary information indicates that gas pipelines had been marked as required.
Investigators will look at whether MGE followed state rules in responding to the gas leak reported beforehand. It could take up to six months before a final report is issued.
Rebecca Craven, program director for the Pipeline Safety Trust in Bellingham, Wash., said natural gas explosions are "ridiculously common," with 37 percent of the most serious incidents caused by digging damage.
"Distribution lines are everywhere in every big city," Craven said. "They get dinged by construction folks or people putting in a new mailbox all the time."