Firefighters first went to the area of Tulare and Fulton mall around 3 a.m. for a fire at the old JC Penney building.
Officials say transients sparked the flames in the basement, while burning away insulation from stolen copper wire. Crews quickly contained the fire in the vacant building, but then they discovered the natural gas leak.
"Because the gas had migrated through multiple sewer lines, Fresno irrigation lines, moving underground, coming up through manhole covers and into other buildings, it posed an extreme explosion hazard as well as an ignition hazard for fire," said Battalion Chief Chuck Tobias.
Authorities closed several streets and evacuated surrounding buildings, Including Chukchansi Park. About ten workers were there at the time, along with some people who lined up early for tonight's bobblehead giveaway.
The entire area re-opened Friday afternoon after crews stopped the leak and made sure the gas levels were no longer a threat.