THE INSIDE STORY of how a top P.I. believes WHITNEY HOUSTON was killed by drug dealers to whom she owed as much as $1.5 million.

The ENQUIRER has learned exclusively that PAUL HUEBEL, a former Chicago police investigator and now a top Hollywood private eye, plans to pres­ent his secret dossier to the bureau’s Chicago office – revealing shocking evidence that details who killed Whitney and WHY.

In a sensational disclosure that can crack the case wide open, Huebl says he’s received a tip from knowledgeable sources that Whitney’s killers are actually caught on hotel videotape, and he’s contacting FBI agents to take a fresh look at the case he believes local authorities bun­gled or covered up.

“I have evidence that points to Whitney being a victim of high-powered drug dealers who sent thugs to collect a huge debt she owed for drugs,” Huebl told The ENQUIRER.

The veteran investigator believes he is close to learning the identity of the vicious killers. He says they are two men from a group of scruffy hangers-on – unknown by Whitney’s usual en­tourage – who kept showing up in the days before the singer’s untimely death at age 48.

His confidential informants insist that close examination of the surveillance videos from the Bev­erly Hilton hotel, where Whitney died, will show two shadowy characters entering and leaving her suite right around the time she died.

Ever since America’s greatest female singer was found floating face-down in a scalding-hot bathtub on Feb. 11, 2012, Huebl has relentlessly gone after her suspected killers.

“They blended into a group of hangers-on that partied with Whitney at her hotel,” he explained. “The drug dealers and their thugs are from the East Coast with connections in Georgia and New Jersey, where Whitney lived.”

The day before she died, he says Whitney received a delivery of cocaine at the hotel pool.

Around 3 a.m., he adds, she partied with a small group in her suite – among them were the drug thugs. After they left, a stoned Whitney was seen on her hotel balcony shouting, “I’m tired of this s***!”

Although no one ever explained what Whitney meant, Huebl told The ENQUIRER: “She was say­ing she was sick of being harassed about money she owed – as much as $1.5 million.

“Whitney refused to pay and argued with the thugs. The next day, they re­turned and killed her.”

With evidence collected through his in-depth investigation and from the coroner’s report, Huebl has been able to re-create Whitney’s death strug­gle in dramatic detail.

Around 2:45 p.m. on that fateful day, her assistant left to run errands. Whitney was left alone in her room, planning to take a bath before attend­ing a pre-Grammy party that night. When the assistant arrived back at the suite around 3:35 p.m., Whitney was dead.

“I have evidence that in that 45-minute time frame, one or more of the hangers-on returned to Whitney’s suite and got into her room,” said the investiga­tor. They demanded the money she owed them, and headstrong Whitney refused to pay. And that’s when “things got physical,” Huebl told The ENQUIRER.

“The coroner’s report shows plenty of small injuries. Whit­ney was thrown into the bathtub and drowned. It seems things got out of hand and rather than just scare Whitney, they ended up murdering her.

“The dealers took whatever they could grab – money, drugs and jewelry – and fled before Whitney’s assistant returned to the room.

“The hotel has security cameras in the hallways and on elevators, and I’ve been tipped off that the killers can be seen on those videotapes,” said Huebl. “The FBI needs to closely examine those security tapes.”

Huebl is stunned that the Los Angeles coroner’s report concluded there was no foul play involved. He insists there is an abundance of shocking evi­dence pointing to murder.

“Whitney’s body shows classic defense wounds which would have occurred while she was battling for her life,” he noted. “There are bruises on her arms and shoulders, a cut on her upper lip, scrapes to her nose and forehead, and lacerations to her scalp.

“Whitney had so many injuries it looked like she had been in a boxing match.

“The coroner’s report confirms Whitney had co­caine, marijuana and sedatives including Xanax in her system. She was so stoned she would not have been able to put up much of a struggle.

“She was planning a comeback and was going to attend a party that night with music industry people she wanted to impress. Why would she take sedatives?

“The water in the tub tested at 93.5 degrees at 9:35 p.m., six hours after Whitney’s death. She had a burnt section of skin measuring 5-by-3 1/4 inches on her lower back. That water must have been scalding hot when she entered the tub. No one would voluntarily get into near-boiling water like that. She was thrown in and her head was held under the water until she drowned!”

The coroner’s report concluded: “Death was due to drowning due to effects of atherosclerotic heart disease and cocaine use.”

Huebl is outraged and says he’ll present his “murder case” to impartial FBI agents working at the Chicago bureau currently headed by Thomas R. Trautmann.

“I believe the Beverly Hills police and the Los An­geles coroner are happy to sweep Whitney’s death under the rug, calling it accidental and closing their investigation,” he said.

“Also, I was tipped off by sources in the drug-dealing world that people from New Jersey and Georgia were involved, and that’s why we need a federal investigation. It’s a matter of an interstate conspiracy. I’m convinced Whitney Houston was murdered. Now it will be up to the FBI to prove it.”