The search for a man who disappeared from a music festival in Michigan is now in its third month – and some of those close to him think he might’ve joined a religious cult.

Kevin Graves, 28, of Highland Township vanished on July 1 from the Electric Forest Festival in Rothbury, an eight-day, two-weekend event featuring electronic music and jam bands on the 2,000-acre grounds of the Double JJ Resort.

Graves had an argument with his girlfriend and told her he was returning to his tent to rest, but he was nowhere to be found later, according to the Oakland County Sheriff’s Department. But his sister, Kellie Farley, now believes he might be alive and living with a fundamentalist Christian group called the Twelve Tribes that some believe is a cult, MLive.com reports.

“It’s hard for us to believe, because he’s not the type of person to just up and disappear that way,” Farley, 34, told the website.

Farley said she’s received nearly 100 messages in the past several weeks speculating that her brother might have joined the insulated group – formed in Tennessee during the 1970s – that has an estimated 3,000 members, including communities in New England and Canada.

The group’s recruiting tactics include inviting people onto one of two distinct double-decker buses, vehicles that have been spotted at similar musical festivals for the past several decades, particularly those featuring acts such as the Grateful Dead, Phish and Bob Dylan.

“People had messaged me that were at the festival and said, ‘Oh yeah, I saw one of these [buses] up there,’ ” Farley told the website. “ … My gut was like, this is it.”

Farley said she contacted the group via its toll-free number on its website and was told that no one had reported seeing Graves. She plans to make additional calls this week but is less convinced that Graves had joined a cult than she initially was.

Authorities, meanwhile, said there’s no evidence to support the theory that Graves joined a cult.

“I have no evidence to support in any way that anything nefarious happened to him,” Oakland County Sheriff’s Sgt. David Bach told the website. “My feeling is he walked away from there; his goal was to be a part of the lore of that festival, of people who disappeared.”

Bach said Graves discussed his intentions prior to the leaving for the festival.

“What money he had – he drained pretty much all of his banks,” Bach said.

Graves’ girlfriend at the time told authorities he had been acting “odd all week,” Bach said, and accused her of having a sexual relationship with another friend at the festival. Sources told MLive.com Graves also had issues with alcohol and substance abuse and had been treated for bipolar disorder.

Farley said her brother – who identified as an atheist – stopped taking his prescribed medication roughly two months before his disappearance because it made him lethargic. And another friend who attended the festival said he doesn’t believe Graves joined the Christian group because it condemns drug and alcohol use.

Graves also spoke about his desire to grow marijuana, friend James Tierney said.

The father of Graves’ former girlfriend, meanwhile, had a blunter belief about what happened to the man who lived in the same house as he did for 18 months.

“He walked away with a plan in mind,” Richard Mobley told MLive.com. “I think he purposely did it to make people think he was dead.”

Anyone with information about Graves’ whereabouts is asked to call the Oakland County Sheriff’s Office at (248) 858-4950.