Police believe Steven Pladl, 45, was watching the home Katie Pladl, 20, was at with her adoptive parents from the parking lot of the Wingdale Liquor Store on April 12.

UPDATE: Warrant: Knightdale incest couple’s infant was found dead in bathroom closet

Four people are dead in three states after a deadly crime spree, at the root of which is an incestuous relationship.

RELATED: Multiple 911 calls released in triple murder-suicide involving Knightdale incest couple

New Milford police said Steven Pladl killed an infant he fathered with his own daughter Katie Pladl, then killed his daughter and her adoptive father, Anthony Fusco, 56.

In a news conference on April 13, New Milford Police Chief Shawn Boyne revealed that Steven Pladl used an assault-style weapon similar to an AR-15 to kill Katie Pladl and Fusco.

Police said Steven Pladl picked up his baby from his mother in Knightdale on Wednesday night.

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“My son just called me and told me, oh god, you know, he killed his baby and he’s in the house,” said Steven Pladl’s mother in a frantic 911 call.

His mother told police that he was headed north to find the baby’s mother, Katie Pladl. Steven Pladl told his mother that he and the baby were going to communicate with Katie Pladl through Skype.

“His wife broke up with him over the phone,” Steven Pladl’s mother said in the 911 call. “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.”

TIMELINE: Knightdale incest couple

However, police reported that the 7-month-old baby was found in Steven Pladl’s empty home. The cause of death has not been released.

Boyne said Steven Pladl had contact with his daughter’s adoptive New York family around 8 p.m. on Wednesday.

Steven Pladl drove nine hours and 500 miles north to the Gaylordsville section of New Milford.

Around 8:40 a.m. on April 12, Katie Pladl and Fusco were found shot and killed inside a Toyota pickup truck at the intersection of Routes 55 and 7 in New Milford.

“A vehicle bearing [North Carolina] registration plates pulled up alongside their vehicle and discharged several rounds from an assault-type rifle into the passenger compartment of the victim’s vehicle,” Boyne said.

RELATED: Knightdale incest couple, baby dead in multi-state crime scenes

Emergency calls released on Friday revealed that an off-duty firefighter from New York witnessed the shooting. He was the first to call 911 and report what happened.

“The car pulled up, went around him and shot him,” the caller said. “A whole clip into his head.”

The firefighter reported both people in the vehicle were dead and that the suspect was driving a light blue minivan.

“It was definitely an out-of-state plate heading into New Milford,” the caller said.

Shortly afterward, Steven Pladl was found dead in the minivan across the state line in Dover, New York, in what police suspect was a suicide. They believe he shot himself. The scene was five miles from the shooting scene in New Milford.

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“[Thursday’s] events were sad and devastating,” said Knightdale Police Chief Lawrence Capps. “We may never understand the mindset or motives of Steven Pladl, but we do know his actions have shattered the lives of countless people.”

Police said Katie and Steven Pladl were father and daughter. They were recently charged with incest and adultery after they reportedly married and had the child in 2017.

According to warrants, Katie Pladl was born in 1998 to Steven Pladl and her mother, then legally adopted out of state.

When Katie Pladl turned 18, she used social media to reach out to her biological parents, the documents said.

The warrants said Steven Pladl left his wife to be with his daughter. They lived in Virginia and eventually North Carolina.

Their child was born in September 2017.

After the father and daughter’s arrest for incest, the baby was placed in the custody of Steven Pladl’s mother, Knightdale police said.

“He killed his wife, he killed her father,” Steven Pladl’s mother said in the call. “I can’t even believe this is happening.”

Anyone with information about what happened is asked to call New Milford police.

The FBI is also investigating.

CBS 17 contributed to this report