NEW MILFORD, Conn. -- 911 calls placed in North Carolina and Connecticut are revealing more details about the events that led up to a multi-state murder-suicide allegedly perpetrated by a man who had a child with his adult biological daughter. The Thursday rampage left all three are dead, along with the woman's adoptive father.

Police in Knightdale, N.C. say they responded to a home around 9 a.m. where Steven Pladl, 45, and his biological daughter Katie Pladl, 20, were living when they were arrested on incest charges out of Henrico County, Va. in January. There, they found their son, 7-month-old Bennett Pladl, dead in a suspected homicide. No one else was in the home.

About 20 minutes earlier, 600 miles north in New Milford, Conn., police responded to a report of a shooting and found a man and a woman dead in a pickup truck with New York plates, the victims of a double homicide. Police developed information on a suspect vehicle, and when New York state troopers found the minivan several miles away across the state line in Dover, the suspected gunman was found inside -- dead of an apparent self-inflicted gunshot wound.

Speaking at a press conference Wednesday, Knightdale Police Chief Lawrence Capps confirmed the victims killed in the pickup truck were Katie Pladl and her adoptive father 56-year-old Anthony Fusco, and that the man who killed himself in New York was Steven Pladl.

Knightdale police confirmed to Crimesider that Steven Pladl is suspected in the death of his infant son in North Carolina.

New Milford police said Friday that the gunman opened fire with an assault-type rifle from a car and shot the victims multiple times in the head and upper torso. Police believe he apparently used the same weapon to kill himself. Police say it wasn't clear why the deadly encounter happened at the New Milford intersection, but police believe it's possible the two victims were in the area to run an errand and the gunman confronted them because their car had slowed.

"Someone just went by and shot this guy in the truck," a stunned witness, who said he was a New York state firefighter, told a 911 dispatcher in a call obtained by CBS affiliate WNCN.

The caller says the victim's car was at a stop sign when the gunman, in a blue minivan, drove around two other cars, pulled alongside it and fired into the truck and then fled. He said two people were dead.

"We need the police, we need everybody, there's two people in the car," he continues.

The alleged relationship between the father and daughter began two years ago in Virginia, where Katie Pladl was born in 1998. Steven Pladl and his wife at the time gave Katie up for adoption shortly after she was born, but in 2016 at age 18, Katie located her birth parents and began living with them in their home west of Richmond, WNCN reported.

When Steven Pladl's wife found out he was in a sexual relationship with Katie and she was pregnant, they divorced and the woman contacted police.

Katie and Steven Pladl reportedly had the baby together in September 2017, married and moved to Knightdale, North Carolina, where they were arrested in January.

They were both charged with incest, adultery, and contributing to delinquency and bonded out of jail. Steven Pladl's bond conditions were modified this month to allow him to travel to North Carolina, reports WTVR.

Katie Pladl's bond terms required her to live with her adoptive parents, who live in New York, and stay away from Steven Pladl, the station reports. She was allowed to travel out of state.

Capps said during a Wednesday press conference he wouldn't comment on the cause or manner of the child's death in North Carolina.

Capps said it was Steven Pladl's mother who called police Thursday morning and requested the welfare check on the infant. Capps said Steven Pladl had been living at the Knightdale residence where the boy was found dead, and that his mother had been living with the child in another home in the community while the incest case was pending trial in Virginia.

The mother said Steven Pladl had picked the child up from her Wednesday night. She told police when she requested the welfare check that she was concerned over comments her son had just made to her on the phone.

In that 911 call, obtained by WNCN, Steven's mother told the dispatcher her son said he had killed the baby.

"He left the baby dead," the caller said. "He told me to call the police and I shouldn't go over there."

The caller then said Steven admitted to killing his wife and her adoptive father, the station reports.

"His wife broke up with him yesterday over the phone," Steven's mother said in a 911 recording. "She's in New York and he told me he was on his way and after bringing the baby to her and then he was coming back."

The mother becomes emotional, saying, "He killed his wife, he killed her father, and -- I can't even believe this is happening."

Capps said it was his understanding Steven Pladl drove to the New York area "for the specific purpose of making some confrontations."

He said his department has been working throughout the day with multiple jurisdictions in Connecticut and New York to try to piece together a motive and timeline of events.

"Obviously we're very saddened by today's events," Capps said. "Events like this are not common in our community. Unfortunately, they are not uncommon in society. We are heartbroken, saddened over the death of this child, and like you, we're trying to make sense of all of the factors that led up to this senseless taking of life."

© 2018 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.