N By Alice McQuillan, Ralph R. Ortega and Dave Goldiner

EW YORK (KRT) – Fearing a new rap war, police brass ordered cops Thursday to watch over hip hop figures following the mysterious killing of Jam Master Jay, law-enforcement sources said.

The clampdown – engineered by a new NYPD unit that monitors the hip hop world – came as cops struggled to find a motive for the slaying of the Run-DMC disk jockey, who had a squeaky-clean reputation and no history of violence.

Cops were watching “a number of people with connections to Jam Master Jay who might possibly be in jeopardy of being targeted,” a police source said.

“I just want to know who killed him and why,” said Bonita Jones, the rap star’s sister, during an interview in their childhood home in Hollis, Queens. “Why did they do this?”

Detectives combed the business records and computers of the 24-7 recording studio owned by Jay, whose real name was Jason Mizell.

The cops also probed the possibility that rivalries between New York and California rappers could have triggered the shooting.

But friends insisted Mizell was not involved in such feuds.

“This has nothing more to do with so-called East Coast-West Coast violence than the sniper murders in Washington did,” said Russell Simmons, the rap impresario whose brother is Run-DMC member Joseph (Run) Simmons.

Police said Mizell, 37, was playing a video football game with pal Uriel Rincon, 25, and a woman when someone was buzzed into the second-floor studio in downtown Jamaica.

The woman told cops she was told to lie facedown on the floor by a gunman who shot Mizell execution-style in the head with a .40-caliber handgun.

The groundbreaking rap star died wearing tan jeans, a brown leather hat, Cartier sunglasses and his trademark white Adidas sneakers, friends said.

Rincon was shot in the leg.

He and the woman offered conflicting stories about the shooting, according to cops. The woman said there were two assailants, but Rincon said there was one gunman – who wore a mask.

Rincon was released from a hospital Thursday but was too frightened to cooperate with cops, sources said.

Three other people – part of an up-and-coming rap group named Rusty Waters – were recording in the studio but didn’t see the attack, cops said.

Police found a gun in a parking lot near the Merrick Blvd. studio but said it was not the murder weapon. They are investigating whether it might have been dumped by the attacker.

The studio has a video surveillance camera, but cops said it did not record the attack.

Investigators also found an unknown amount of marijuana inside, a police source said.

With fear spreading among some hip hop artists, rapper 50 Cent canceled a show Thursday night in Manhattan after reporting what cops called a credible threat.

Police offered to provide uniformed cops to guard 50 Cent, but he refused the offer and decided to leave town instead.

Detectives are investigating whether other rappers felt insulted by a song on 50 Cent’s latest record, which was produced by Mizell.

But they stressed they have no evidence the so-called dis had any connection to the shooting of Mizell.

Outside the studio, a memorial grew as fans brought flowers, candles and even a pair of Adidas sneakers – a reference to Run-DMC’s smash hit “My Adidas.”

“Nobody deserves to die like that,” said Natacha Matthews, 30, of Long Island, N.Y. “He believed in being genuine. His whole aura was positive.”

A mile away, at the aluminum-sided home where Mizell grew up, his sister closed her eyes and nodded her head to the rhythm of “It’s Like That,” one of her brother’s biggest hits.

Jones said she spoke to Mizell hours before the shooting, but for some reason they forgot to end the conversation with their usual “I love you.”

“He was the best brother I ever had,” Jones said. “I loved him very much.”