Authorities have remained tight-lipped about a motive ever since Jace Crehan and his girlfriend, Brittany Monk, were arrested a month ago in the stabbing death of her stepfather, who was found stuffed in a plastic bin in his Zachary trailer.

But now Crehan is speaking out from jail, suggesting he killed Robert Noce at least in part because the man had raped Monk for years and received a light sentence after he was convicted of sexual improprieties with her.

Noce repeatedly assaulted Monk throughout her childhood, she claims. Monk’s locked up, too, and is pregnant with Crehan’s child.

“I’ll say this, I Jace Crehan, killed Robert Noce and he admitted in his final moments his rape against Brittany,” Crehan wrote in a letter.

In gushing handwritten notes and recent phone calls to The Advocate, Crehan described how Monk’s history with Noce drove him to desperate efforts to defend her, comparing his actions to Gary Plauche, who avoided prison time after killing his son’s accused molester at the Baton Rouge airport in 1984. Crehan also recounts stories about his girlfriend’s anxious childhood, one in which home life tangled with booze and abuse in the small town outside Baton Rouge.

“This love is beyond what one would feel for their wife or child or family member,” Crehan wrote. “It was something that overcame me. I couldn’t control this enormous amount of obligation. I felt indebted to her. I was more than just her boyfriend, fiance, lover. I was her guardian, her protector, her hope.”

Lawyers for Crehan and Monk declined to comment. Noce’s family members — and attorney Drew Louviere, who represented Noce when he was accused of rape — didn’t respond to queries.

After facing a count of aggravated rape against Monk in 2013, Noce pleaded “no contest” to carnal knowledge of a juvenile on June 22. That plea functions as a guilty plea but allowed him to maintain his innocence. A transcript of that hearing shows the prosecutor in the case said the plea deal was Monk’s decision.

East Baton Rouge Parish District Attorney Hillar Moore declined to speak about the plea negotiations.

Noce served no jail time and was sentenced to five years active supervised probation.

What was perceived as a light sentence unnerved the now-accused couple, Crehan said, adding that the many months of court proceedings had grown exhausting.

After the June 22 plea deal, Monk suffered increasingly violent night terrors, Crehan said.

“One night it was worse than ever and it was on that night that I came to the conclusion that if I didn’t do something immediately, (Noce) might actually end up killing my family, in some way,” Crehan wrote.

Still, he said in a phone call, “it was never my intention to do what happened,” adding that he’s “not a violent person.” He said he was provoked after Noce tried to attack Monk but stopped short of explaining exactly how or why he and Monk showed up at the Zachary trailer. A police affidavit of probable cause says Crehan admitted “that he and his co-defendant entered the victim’s residence without permission.”

Crehan presents a narrative with echoes of the Baton Rouge homicide of Jeff Doucet, a karate teacher who had confessed to molesting Gary Plauche’s son in 1984. The assassination, caught on camera by WBRZ-TV, happened while Doucet was being escorted by deputies back to Louisiana, after he’d been found with Plauche’s missing 11-year-old son Jody in a California hotel room. Wearing a disguise, Plauche showed up at the Baton Rouge Metro Airport after hearing of Doucet’s arrival, idling near a phone booth before firing a single shot into Doucet’s head.

Plauche, who pleaded no contest to manslaughter, was ultimately sentenced to five years of probation and hundreds of hours in community service. He served no prison time.

What’s unclear is whether law enforcement sees Crehan’s and Monk’s case as similar in any way to the Plauche saga and whether an investigation into the timing and planning of the killing would reveal another side to Crehan’s account. The East Baton Rouge Parish Sheriff’s Office, which is investigating the incident, declined to comment on that aspect of the probe.

Crehan said he met Monk on Instagram in early 2014, when she was 16, after they began flirting though comments on the social media platform. “Needless to say I fell in love with her immediately,” wrote Crehan, a former car dealership employee who was a Hahnville volunteer firefighter.

Monk began opening up, telling him how her parents were frequently out at the bar and how she never felt safe because her guardians invited “a bunch of strange drunk men” to the house, he said. Then still a minor, she’d been living with her mother in Walker after moving out of Noce’s trailer in Zachary, where she told police she’d spent her childhood being sedated and raped by him, according to police reports. She had nightmares, but they subsided when Crehan slept next to her, he said.

“So it was at that point that I felt it was necessary for me to be there all the time,” he wrote.

A letter to Monk in jail yielded no response.

Monk’s mother had left her and her younger sister behind after a breakup with Noce, according to police reports. Love letters obtained by The Advocate show Monk’s mother growing increasingly plaintive toward “Robbie” around the time Monk was 4.

“Growing up I never had anything. I had a really hard life. Love is not the problem now because I know you love me, it’s the fact that you think your T.V. is more important or just sitting there holding me is to(o) hard for you,” the mother wrote to Noce, in cursive.

Monk’s mother didn’t respond to multiple requests for comment.

Monk appears to have few close family ties, though her older half-sister in Amite has created an online campaign to raise money for Monk’s bail and medical expenses.

Katelyn Marcotte, 19, said she’s only known Monk for about three years after discovering they share a father but spoke highly of her sibling, who was a “straight-A student,” she said, and is “very, very smart for her age.”

Monk looked after her own mother’s other young children much of the time, Marcotte said. Monk’s child, due in the fall, will be named Vaan, Crehan said.

Crehan wants Monk to be freed, writing that “she is innocent,” though police said her DNA was found on the drum containing Noce’s body.

“Now that (Noce) is dead he can no longer hurt anyone else, especially my family. Brittany has never felt safer than she does with me and is now no longer afraid,” Crehan wrote.

Crehan’s father, Layton Crehan, wonders whether his son fully comprehends the potential fallout of his actions.

“He’s 21 years old, and he’s probably never gonna see the light of day again,” Layton Crehan said. “He seems to think he ain’t gonna get much time for it and whatever else. That’s not the way the world turns around anymore.”

The killing “caught me out of nowhere,” Layton Crehan said. “He’s always been a ‘yes, sir,’ ‘no ma’am,’ ” kind of guy, he added. He and his wife have been talking about eventually adopting Monk’s child, he said.

“I wish he would have come to me first before he’d done it,” Layton Crehan said. As to how he would’ve advised his son to act, he responded, “I’m not exactly sure, but it wouldn’t be what happened.”

Editor’s Note: This story was changed Aug. 9, 2015, to show that Jace Crehan and Brittany Monk plan to name their baby Vaan.

Advocate staff writer Danielle Maddox contributed to this story.

Follow Maya Lau on Twitter, @mayalau.