The North Carolina incest dad, whose Thursday murder spree took the lives of the biological daughter he married and the infant child he fathered with the girl, reportedly explained his possible motive to his own mother in a stunning phone call moments before he turned the gun on himself.

In a 911 call obtained by WNCN in Raleigh, Steven Pladl's mother -- who attended the lakeside "wedding" between her son and his daughter-turned-wife -- said Steven admitted to killing 20-year-old Katie Pladl and the couple's 7-month-old son, Bennett. Pladl's mother had been taking care of the boy since his parents were busted earlier this year for the illegal relationship.

“Oh God, he told me to call the police and I shouldn't go over there,” a caller identified as Steven's mother told the dispatcher.

She added: “He killed his baby and [the baby is] in the house...His wife broke up with him over the phone yesterday...I can't even believe this is happening."

Steven Pladl, 42, was found dead Thursday morning in his vehicle in Dover, N.Y., several miles from the New Milford, Conn., murder scene where Katie and her adoptive father, 56-year-old Anthony Fusco, were discovered. Those two were also found dead in a vehicle that had one of its windows shot out, police said.

The child’s body was found inside Pladl's home, hundreds of miles south in Knightdale, N.C., the same day.

Pladl’s mother -- the one who tipped off authorities about the toddler's death -- had obtained custody of the child Steven fathered with his daughter following their arrests in January for adultery and contributing to delinquency, police told WNCN. The mother lives in Cary, about a half hour’s drive from Knightdale, and reportedly told them that Steve had picked Bennett up from her home Wednesday night.

The toddler was found dead around 9 a.m. Thursday after officers performed a welfare check on the Knightdale home, where Pladl -- and previously his daughter -- had lived.

Katie Pladl moved out of the home earlier this year and was living with her adoptive family in New York after bonding out of jail in Virginia, WNCN reported, citing court documents.

It appeared Steven Pladl drove through the night Wednesday to hunt down his daughter and Fusco. They were found dead around 8:40 a.m. Thursday, and Pladl shot himself shortly afterward.

"This is a terrible tragedy that nobody foresaw,” Rick Friedman, an attorney who had been representing Steven Pladl, told WNCN. “I really believe that if the judges or the prosecutor or the defense attorneys in this case had any clue that the minor child or anyone would be harmed there would not be a bond set for any of the parties.”

While the couple's recent breakup may have spurred on Thursday's murders, the exact motive for the shooting is under investigation, New Milford Police Lt. Lawrence Ash said.

Following their January arrest, Steven Pladl was let go on a $1 million bond but his daughter remained at the Wake County Detention Center until late February, when she posted a $12,000 secured bond, the Daily Mail reported. According to the terms of her release she was prohibited from contacting Steven and was mandated to reside in New York with her adoptive parents.

Pladl and his first wife, Alyssa, reportedly gave up Katie for adoption as an infant in the 1990s but they all reunited in 2016 after Katie tracked down her biological parents via social media.

Pladl and Alyssa divorced in 2017. Around that time, he apparently became romantically involved with his biological daughter.

In July, an image posted to an Instagram account purportedly belonging to Katie showed a heavily-pregnant Katie kissing Steven Pladl.

“Nothing fancy, just love,” the post had read, with the hashtags “#justmarried #simplewedding #pregnancy”.

The lakeside event was attended by Pladl’s mother and Katie’s adoptive parents, according to the Daily Mail and photos posted on social media.

Katie Pladl gave birth to Bennett in September.

“There are no words to describe the sense of betrayal and disgust I'm feeling,” Alyssa told the Daily Mail in February. “I waited 18 long years to have a relationship with my daughter -- and now he's completely destroyed it.”

Alyssa – who ended up having two more children with Steven – found out about the relationship between Steven and Katie after reading entries in a journal her 11-year-old daughter had been keeping, the Daily Mail reported.

The website said after Alyssa and Steven split, the daughter had spent time living at Pradl’s Knightdale home and wrote about Katie being heavily pregnant. She also referred to Steven as “Satan”.

During her time in New York, where she was formally known as Katie Fusco, she graduated from Dover High School in 2016, the Poughkeepsie Journal reported.

The newspaper added that, in an online artist portfolio page belonging to her, Katie wrote about attending Dutchess Community College and her plans to “hopefully be graduating from SUNY Purchase” in 2020.

“After that, although hard to tell at this time, I plan to already have started my career in digital advertising, some small projects like logos to larger ones like signs or websites” the page said. “I will hopefully gain attention to bigger businesses and make a name for myself.”

Katie also reportedly wrote about becoming “nearly famous” at Dover High School for writing comic strips.

“A pen and something to draw on became a safe place for me,” the page said. “Ink became my weapon against rules and regulations. There wouldn’t be a corner in a classroom or park that didn’t have a secret little character living on it…to be short: for me, a life without art is no life at all.”

Fox News’ Matt Richardson contributed to this report.