Mayor Sly James declined to say whether the person was a man or a woman.
Authorities have been looking for a woman who worked at JJ's restaurant, was seen there before the Tuesday evening blast and was reported missing afterward. They previously said she was the only person still unaccounted for following the explosion and fire.
But James said at a news conference that authorities can't be "100 percent sure that we can account for every single person that may have been at JJ's when the explosion occurred."
Crews have been searching the site feverishly ahead of a major winter storm bearing down on the city. James said 15 people were injured in the blast. Six were still hospitalized Wednesday morning.
The blast occurred after a construction crew apparently struck a natural gas line. The explosion was felt for nearly a mile around the restaurant, shattering glass in nearby buildings and sending an ominous smoke plume above the city's prized outdoor shopping district.
One of two people first feared to be missing was later found at a hospital. But the woman who worked at JJ's was still missing, and James had stressed that finding her remained the primary focus of Wednesday's efforts.
"We have a major storm coming in this evening," James said. "We're going to work diligently to get in (to the blast site) to get underneath that weather."
JJ's was a beloved fixture on the city's culinary scene for more than 27 years. Locals knew it as a prime after-work stop, though the restaurant won a broader reputation after it consistently received high ratings from contributors to Zagat's restaurant guides, both for its food and its list of hundreds of wines.
The shopping area was established in 1922 by J.C. Nichols. Based on the architecture of Seville, Spain, it includes stores, restaurants, apartments and offices.