The couple was in the one-story concrete block building covered with brick when the fire started in an area they were using as living quarters, Nichols said.
Nichols, the first to arrive at the store from his home less than a mile away, said fire had engulfed 60 to 70 percent of the building.
The firefighter who was injured was part of an interior attack team that was cut off from its exit for a couple minutes in a "very, very hot fire," Nichols said.
"They were bailing out of windows ... ," Nichols said of the team's unconventional escape.
The concrete block construction helped create an oven-like effect that elevated the temperature, Nichols said.
After retreating, firefighters went into a "defensive mode" to get the fire under control and ultimately cut a large hole in the building's tin roof to ventilate the structure, which retained heat, Nichols said. Though heavily damaged, the roof remained intact, Nichols said.
Barker and Berkeley County Sheriff's Department Lt. Gary Harmison said the owner was so upset about the fire they couldn't talk to him about what happened.
Harmison leads the county's fire investigation team, which also responded to the fire.
Assisting investigators and Hedgesville firefighters were crews with Back Creek Valley Volunteer Fire Department and paid staff with the City of Martinsburg and the Berkeley County Department of Homeland Security and Emergency Management, Nichols said.
The last responders left the scene at 12:20 p.m., Nichols said.