The three-alarm fire sparked at a boarded up building at Fresno Street and Pontiac around 3 am.
Firefighter Robert Lee captured helmet cam footage while heading up a ladder to help spray water and foam on the roof of a home next door that was getting ready to ignite, moments before the explosion.
It soon prompted a mayday call.
"When you first hear that, anything can be possible. That could been someone was injured or killed in the line of duty," Lee said.
He said his first instinct was to help his fellow firefighters, but his job was to make sure no one else got hurt.
"Even if we hear a mayday, we have to stay task-oriented and put the fire out," Lee said.
Lee said the call ended up being for his captain, who was knocked down by the force of the explosion and hit with debris, but is okay.
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A mayday call, however, is rare.
Fire officials say the last time they sent one out was for Captain Pete Dern, who fell through the roof during a garage fire several years ago leaving him with permanent injuries.
"When you hear mayday, that's the worst-case scenario. Those are the words you never want to hear. Sometimes this situation happens in a blink of eye, something that's not anticipated can happen," Fresno City Fire's Shane Brown said.
Investigators now will be working to figure out how the fire started and what may have caused the explosion.