The surviving 13-year-old boy escaped through the garage as his father fired a rifle at him, the Polk County Sheriff's Department said.
Investigators said Troy Ryan Bellar fatally shot his wife, Wendy Bellar, 31, and their 5-month-old and 8-year-old sons before killing himself late Sunday in Lakeland, between Tampa and Orlando. They said they had not found any suicide note.
Deputies believe Wendy Bellar was trying to leave the home with the three children following a dispute just before the shooting happened, reports CBS affiliate WTSP-TV in Tampa.
"I heard Wendy screaming and the baby crying," said next-door neighbor Doug Lane. He described hearing a barrage of gunfire and Wendy Bellar sounding as if she was in "horror, terror."
Troy Bellar opened fire with a 7.62 mm rifle, killing Wendy, 5-month-old Zack James Bellar and 8-year-old Ryan Patrick Bellar in the front porch area.
"It's a very, very angry and selfish and cowardly person who murdered his family," Wilder said. "What motive or what explanation can you give?"
Wilder said Troy Bellar chased 13-year-old Nathan Bellar through their garage, but tripped over a bicycle and fell. The father's gun discharged, but he got back up and fired at least two more rounds before killing himself.
Nathan made it safely to a neighbor's house and was being placed with family members.
"He's very upset but he was a brave young man, was able to have an extensive conversation with our detectives and help us to learn what happened," Wilder said.
Troy Bellar is the boy's father, but Wendy Bellar is not his biological mother, Wilder said.
No neighbors were injured by gunfire in the quiet suburban community, where the Bellars lived in a ranch-style home with green shutters and a screened-in front porch.
Investigators say Troy Bellar ran a home remodeling and handyman business. Wendy was a stay-at-home mom.
Lane said Troy Bellar's business seemed to keep going despite the economic downturn.
"He said the remodeling had dropped off, but he was doing repair work for commercial apartments and that sort of thing," Lane said. "He said that he was staying busy."
Wilder couldn't say if the family was experiencing any financial difficulties.
Records show the husband had been arrested twice in the county, for armed robbery and aggravated battery in 1994 and for driving under the influence in 1999. He was convicted of petit/retail theft in the first case and of driving under the influence in the second.
Wendy Bellar was arrested in March on suspicion of domestic battery.
According to an arrest affidavit, Troy Bellar accused his wife of hitting, punching and scratching him. He was uninjured and no motive was provided.
The report said the couple had been married for 10 years. The case was pending.
Lane said he never heard the couple arguing. He said he saw them sitting outside together Sunday evening, giving no indication that anything was wrong.
"Troy and Wendy were good, good neighbors," Lane said. "Fortunate to know them. Just an awful shock."