A firefighter lost three children in an early morning blaze that tore through a home day care in Pennsylvania, killing five and leaving a woman injured, authorities said.

Joe Crotty, chief of the Lawrence Park Volunteer Fire Department, said firefighter Luther Jones was responding to another fire when the blaze ignited at Harris Family Day Care in Erie, NBC News affiliate WICU reported.

The victims, who were not immediately identified, range from 8 months old to 8 years old, authorities said. A fourth child who died was a sibling of Jones' three children, but had a different biological father, fire officials and family confirmed to NBC News.

Jones had no idea his children were at the overnight day care center a few blocks away, WICU reported.

It's "unthinkable," Crotty said. "It's really beyond comprehension."

Paul Laughlin leaves stuffed animals outside the scene of a house fire that left multiple people dead in Erie, Pennsylvania, on Aug. 11, 2019. Greg Wohlford / Erie Times-News via AP

Earlier, city officials said in a Facebook post that firefighters responding to a structure fire shortly after 1 a.m. rescued seven who were trapped inside a home.

On Facebook, a local firefighters union said it had been a "tough night" for emergency workers who responded to the fire.

Investigators were still working to identify the cause of the fire.

A state-run health and human services website listed Harris Family Day Care at the address where the fire occurred. A message left with a phone number for the business was not immediately returned.

WICU reported last year that the day care's owner, Elaine Harris, had run the facility for 18 years.

Harris was licensed to have six children at a time 24 hours a day, seven days a week, the station reported. She worked with kids who ranged from newborns to 13 years old.

"This is what I was meant to do," she told the station. "I like to get them in early, teach them, instill in them their values before they get out there in the world."

It wasn't clear if Harris is the woman who was injured in the fire.

Fire Chief Guy Santone told The Erie Times-News that a neighbor was also injured.