A $30,000 drug rip-off in California at least eight years ago sparked a simmering feud that investigators believe led to the assassination of legendary rap star Jam Master Jay, The Post has learned.

Since the sensational rubout three weeks ago, investigators and a legion of fans of the slain hip-hop star have struggled to explain why the man born Jason Mizell, who spoke passionately against violence and espoused family values, could have met such a vicious, seemingly unprovoked end.

Numerous law-enforcement sources told The Post they believe the murder stemmed from a deal gone bad involving Jay — who cops say had no experience with drug deals — and the prime suspect in his slaying, Curtis Scoon.

Scoon, at the advice of his lawyer, Marvyn Kornberg, has declined to be questioned by the police.

"The police have all these beliefs and theories," Kornberg said. "Any time they want to arrest my client, we will be happy to produce him. The truth of the matter is, they are barking up the wrong tree."

The 6-foot-4, 250-pound Scoon has been arrested three times, including twice for robbery, but both times the charges were dismissed. His lone conviction was for possessing stolen property, for which he received a conditional discharge.

Representatives for Mizell could not be reached for comment.

Based on interviews with friends, relatives and associates of Jam Master Jay, this is the scenario cops pieced together about what happened:

Scoon and the slain hip-hopper, both of whom grew up in hardscrabble Jamaica, Queens, each reportedly ponied up $15,000 to buy 2 kilos of cocaine from dealers in California and never got the drugs.

Investigators think Mizell — who they said was not a regular in the drug world — might have seen the deal as an opportunity to make a quick buck. His inexperience may have fueled the deal's failure, probers said.

But the people who ripped off the pair were friends of Mizell, investigators said, and Scoon wanted the star to repay his losses.

Mizell refused, insisting he also was a victim.

But Scoon continued harboring a grudge over the years.

"He never let it go," one source said.

About a month ago, cops believe, Scoon, who splits his time between Queens and Georgia, made several threatening calls from the South to Mizell, saying he was coming north.

But Mizell, who was in debt and owed the government hundreds of thousands of dollars in back taxes, sent a couple of his close aides to confront Scoon.

Mizell's pals told Scoon he was putting himself in harm's way if he did not back off, the sources said.

The cops believe Scoon acted scared and agreed to end his demands.

But instead, cops now believe he plotted to kill Jam Master Jay, hiring a hit man and an ex-con to serve as a lookout.

On Oct. 30, the lookout went to the studio. When Jay showed up, the lookout telephoned the hit man, who raced to the studio, cops believe.

With the ex-con guarding the front door, the killer walked up the stairs to the second-floor studio, where Jay was playing a video game with Urieco Rincon, 25, and a singer named Lydia.

The killer pushed Lydia to the floor before shooting Mizell at point-blank range in the head with a .40-caliber handgun.

Rincon was wounded in the leg before the gunman fled.

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