French, 25, died at the scene along Interstate 65 in central Kentucky. His mother, Lisa French, was seriously injured and taken to a hospital in Louisville, said Kentucky State Police Trooper Jeff Gregory.
French's uncle Richard Peters, the chief of the Glendale Fire Department, had also responded but didn't see the semi hit his sister and nephew. The semi also hit a fire truck.
Fighting fires was Jonathan's "pride and joy," Peters said. When the call went out shortly before 3 a.m. EDT Wednesday, Jonathan French was among the first ones to get to the fire station.
"He was one of the ones you could count on," his uncle said.
Firefighting ran in the French family. Jonathan French, a farmhand who was engaged to marry, had volunteered for the department about five years. His mother had been volunteering for about seven years. Firefighting was an initial bond that also brought Jonathan French's parents together, and he had a brother and a sister who volunteer for the 23-member Glendale department.
"It's just something that gets in the family blood," Peters said.
Glendale is located about 50 miles south of Louisville. The community is so small its business district spans just parts of two blocks.
Peters said the crew was about to finish putting out the vehicle fire when it hit Jonathan French, his mother and clipped the front of the fire engine.
"We didn't see anything but flying debris," he said.
The fire truck's emergency lights were on at the time.
The truck driver was identified as John M. Hatton, of Covington, Kentucky. He was not hurt.
Gregory didn't know how many people were in the burning van or if they were hurt.
No charges have been filed.
Volunteer firefighters gathered at the Glendale firehouse Wednesday to mourn their fallen colleague. A U.S. flag in a park next to the fire station was lowered to half-staff.
"We're crying, we're hugging," Peters said. "I guess everybody is kind of numb at this point, still trying to take it all in."
Nearby fire departments stepped forward to answer Glendale-area emergency calls for grieving firefighters.
"It hurts us all because we are a brotherhood. The Glendale Fire Department is immensely devastated at this time," said Elizabethtown Fire Chief Michael Hulsey, whose department is nearby.