EXCLUSIVE: Robin Williams' Suicide 'Riddles' Exposed After 10th Anniversary of Comic's Heartbreaking Passing – Including How His Death Note Was 'Faked'

Top investigators want to reopen Robin Williams’ death case after being left convinced the comic DIDN’T hang himself — and suspect he was murdered by ruthless drug dealers, who staged the crime scene to look like suicide.

After intensive analysis, detectives are also blasting California cops for botching the case, RadarOnline.com can reveal.

The investigators say lawmen ignored glaring evidence the Oscar winner was grappling with shocking secret money problems and still had ties to vicious drug pushers from his days as a cocaine junkie.

"Had the police investigators not closed the case so early they would have seen Robin Williams' death was no accident, but that he was brutally murdered," a source told us.

"And the autopsy ruling that Williams' death was a suicide seems a sinister cover-up just to protect the cops who failed to do their job."

In bombshell revelations, the investigators point to stunning screw-ups by cops, including inexcusable failures to:

• Test for DNA and fingerprints on the belt around Williams’ neck and a knife used to cut him.

• Check for a knockout drug on a towel found wrapped around the belt on the victim's neck.

• Probe Williams' finances and determine if he owed money to his old drug pushers.

Hollywood private eye Paul Huebl said: "When you consider how quickly authorities called this death a suicide without looking into Williams' druggie past, it’s a good idea to reopen the case."

Private homicide investigator Peter Rydyn found an alarming series of red flags pointing to foul play.

While Williams had kicked a long-standing cocaine addiction, "that could have left dealers angry he was no longer a customer, or he quit while still owing them money," he said.

Rydyn noted the comedy genius' ties to greedy drug dealers date back to 1982, when he partied with John Belushi on the very same evening the Saturday Night Live legend died from a cocaine and heroin overdose.

And in an explosive revelation, Rydyn discloses that the 63-year-old star was broke and couldn't pay what he owed.

His fortune had been drained by a pair of divorces and he was having a hard time finding new work due to health woes.

The Mrs. Doubtfire star was so strapped for cash, he’d stopped donating to his favorite charity and was trying to sell his 640-acre Napa vineyard, said Rydyn.

The actor's body was found on Aug. 11, 2014, in the $6million waterfront mansion he shared with third wife Susan Schneider.

He was discovered in his stepson’s bedroom, where the door had been locked from the inside. A belt was secured around his neck, and the other end of it was wedged between the closet door and the door frame.

A pocketknife was on the floor.

The coroner said Williams used the blade to cut himself on his left arm 10 times before dying.

However, investigators say killers could have used the weapon to torture the star to reveal a hidden stash of cash.

The autopsy concluded the comedy genius – who was on anti-depressants and also pramipexole and levodopa for a Parkinson's-style illness – killed himself by asphyxiation, using the belt.

Shockingly, a suicide note said to be found at the death scene has been exposed as a fake – and Rydyn insists the death scene could've been staged.

Now, another private detective, a former technical adviser to the Marin County Sheriff’s Office, has also discovered strong evidence Williams was murdered in his Tiburon home.

The private eye says the powerful meds Williams took would have made it easy to overpower and kill him. And the locked door to the death room was a push-button type that anyone could open with a bobby pin and then re-lock when leaving.

The sleuth also notes Williams had shown NO suicidal tendencies, and there was "little evidence" that he died from a violent death like hanging.