ONEIDA, NY – Early this month, Debbie Mahoney woke from a nightmare.

Her long-lost little sister Tammy, last seen on a tiny Native American reservation in Oneida, NY, in 1981 and believed to have been gang-raped and murdered, came to her in a disturbing dream.

“It’s like she won’t rest until this case is solved,” Mahoney said.

It’s been 42 years since 19-year-old Tammy, a SUNY Morrisville student from Farmingdale, Long Island, was picked up while hitchhiking to a friend’s house.

Cops say she was taken to the nearby 32-acre Oneida Nation Territory in what people up here, including the Natives, still call “Indian country.”

Now, her disappearance and presumed murder is one of the hottest cold cases in the country.

The FBI held a news conference in May saying the case was still active and that several new witnesses have come forward.

Recently, the State Police’s Major Crimes and Cold Case units joined the investigation.

Police told The Post that they think they’ve known who killed her from the start.

At least 13 people were present at a party in a trailer on the Oneida Nation Territory, a small reservation in Oneida which was then a ramshackle, mud-filled stretch of land filled with mobile homes.

Investigators say it was “suggested” to them by these witnesses that Mahoney, who may have known some Oneida Nation members, was gang-raped and killed.

But since the crime allegedly occurred on sovereign Native American land, it’s been difficult to both investigate the case and get witnesses on the record.

At the time of her disappearance, local police were not allowed on the Oneida Territory without permission and there had been violence between the residents and law enforcement at times.

But since the crime allegedly occurred on sovereign Native American land, it’s been difficult to both investigate the case and get witnesses on the record.

“We know who the bad guys are and they know that we know,” said former Oneida Police detective and former Madison County Undersheriff Doug Bailey, 76, who’s been investigating the case for decades. “I hope they still are looking over their shoulder every minute of every day of their life. They’re local, they’re here … I see them in the aisles at Walmart.”

Bailey was so focused on the Mahoney case that he visited Tammy’s mother, Jean, who died in 2020, in Long Island, and his mother and Jean became pen pals.

Jean also took Bailey and his wife to a gravesite near Farmingdale which is still waiting for Tammy’s remains should they be found.

“It’s so hard that she passed without any closure,” Bailey told The Post. “But I’ll tell you there are people around here who should be scared to death right now. There are also people who’ve been carrying around a very dark secret for decades and they should unburden themselves.”

The timeline in Tammy’s case is astonishing for the number of stops and starts, clues, and dozens of so far-fruitless digs in the area that have taken place since she was last seen on May 8, 1981.

Bailey said he and his team investigated more than 900 leads over the years.

One primary person of interest has died and another killed himself, according to Post interviews with multiple sources.