The fire started around 3:40 a.m. Friday on the 200 block of Woodlawn Avenue.
The fire quickly went to a second alarm before it was brought under control.
"It's very hard, it's terrible," said Joseph Shamma, of Hamilton Township. "My heart goes out to the family. I mean I see them every day. We weren't pals but they were regular people...it's horrible."
The Action Cam on the scene showed the home severely damaged by flames.
Three others were taken to the hospital with injuries.
Shamma is a landlord for the semi-attached home where the fire spread. He said his tenants were able to escape due to a smoke alarm.
"The tenant called me, one of them called me, and told me that the house is on fire so I came right away but it's unbelievable," said Shamma.
The Red Cross says three families are displaced, with a total of 10 people.
There is no word on how the fire started.
The Mercer County Prosecutor's Office is leading the investigation. Anyone with information is asked to contact Detective Anthony Sturchio at (609) 989-6406.