A New Jersey man who killed a 7-year-old Girl Scout in 1973 has reportedly died in prison. 

Joseph McGowan was convicted of raping and murdering his neighbor Joan D'Alessandro on April 19 that year after she knocked on his door while selling cookies, according to NorthJersey.com. 

McGowan reportedly lived just three doors down from the little girl and was a high school chemistry teacher. 

"She said, ‘Goodbye, Mommy. I’ll be right back,’" Rosemarie D'Alessandro told News 12 of New Jersey, recalling the last words her daughter said to her. 

After McGowan admitted to the murder, little Joan's body was later found at Harriman State Park in New York state. 

The killer was sentenced to life in prison and was last denied parole in 2009. He would have been eligible for parole again in 2025, according to D'Alessandro’s family, NorthJersey.com reported. 

Joan D'Alessandro, 7, was selling Girl Scout cookies when she was murdered by her neighbor Joseph McGowan in 1973. McGowan died in prison on June 5.

"The first thought that came into my mind is now we could concentrate on the 50th anniversary of Joan’s impactful and loving legacy, which will be 50 years in 2023," D'Alessandro, said. "We won’t have to use the time and energy to fight to keep him in prison."

ARREST MADE IN 1972 COLD CASE MURDER OF ILLINOIS TEEN STABBED 36 TIMES

McGowan died on June 5, the New Jersey Department of Corrections reportedly said. His cause of death wasn’t immediately disclosed. 

The young girl's murder prompted Joan’s Law, which was passed in 1997 in the state. It bans parole for criminals who murder children under 14 in connection with a sexual offense. A federal version of the law was passed the next year. 

Rosemarie D'Alessandro was instrumental in advocating for the law and has helped the passage of other laws related to children’s safety. 

"Joan could’ve been put in a cemetery and left there," her mother said of Joan’s Law. "I didn’t want to leave Joan there. I wanted her to be remembered, to be known. She stood up for others, I was going to stand up for her."

Joan’s Law didn’t apply to McGowan because he was convicted before it was passed, according to NorthJersey.com

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D'Alessandro called McGowan’s parole hearings "torture," according to News 12. 

"I’m so thankful he didn’t make it out – so this way no other children, no other adults can lose their lives and suffer a lot, and that’s what I’m so thankful for," she told News 12.