The firefighter, Luther Jones, was with the Lawrence Park Volunteer Fire Department handling other calls while the neighboring Erie Fire Department battled the deadly flames at the Harris Family Day Care.
His children -- two daughters, one son -- were among those killed. Jones dropped them off at the center before going to work and had no idea about the fire there. His department was responding to a false alarm at the time.
Erie firefighters responded to the blaze in the three-story building around 1 a.m. amid reports of children trapped inside.
The victims, who have not been officially ID'd, ranged from 8 months to 8 years old, authorities said.
Two other children, 17 and 12, climbing on the roof and jumped to the ground. Other children were rescued by firefighters.
Jones has been a volunteer firefighter for 15 years and has been with the Lawrence Park department for about a year, Lawrence Park Fire Chief Joe Crotty told Fox News.
"Just an outstanding young man," Crotty said.
The cause of the blaze has not been determined. Erie Chief Fire Inspector John Widomski told The Erie Times-News that a smoke detector was found in the attic.
Day care owner Elaine Harris was taken to a hospital in stable condition.
The state Department of Human Services Office of Child Development and Early Learning listed the day care as in compliance with requirements following a Dec. 28, 2018, inspection. But a Jan. 3 inspection note highlighted "ashes and cigarette or cigar butts" in "a child care space, play space or food preparation area."
Freelance reporter David Aaro contributed to this report.