NY- The teenage son of Mets great Bud Harrelson has been tied to the coverup of the samurai sword slaying of a retired city cop and will be charged in the gruesome murder, authorities told the Daily News.

Troy (T.J.) Harrelson, 19, helped his best friend, Zachary Gibian, 18, dispose of the murder weapon and bloody clothes after Gibian killed his stepfather in February 2005, prosecutors allege.

Harrelson is now cooperating with police and has agreed to testify against his pal if necessary, according to prosecutor John Collins, chief of the Suffolk County district attorney’s homicide bureau.

“T.J. Harrelson will cooperate,” Collins told The News. “If he satisfies the conditions of the agreement he will plead guilty to one count of hindering prosecution.”

The teen’s famous father, Bud Harrelson, played for the Mets for 13 years, helping the team win its first World Series in 1969. The two-time All-Star may be best remembered for his scrap with Pete Rose during the 1973 National League playoffs; he later managed the team in the early 1990s.

“Harrelson’s father was a good man, but he was always getting T.J. out of scrapes,” said Shirley Nager-Stein, the aunt of Gibian’s slain stepdad.

“Well, this is … helping to hide a murder, so he can’t wiggle out of this,” she said.

On the day of the savage killing, retired Manhattan cop Scott Nager, 51, was asleep on his living room sofa in his Hauppauge, L.I., house when Gibian allegedly crept up behind him with a samurai sword.

Gibian cocked the sharp sword and swung it into his stepfather’s neck, according to prosecutor John Scott Prudenti.

“The victim was able to right himself, and then, in a flash, [Gibian] swings the sword again, with a force so great that he nearly decapitates the victim,” Prudenti said.

In written and videotaped confessions, Gibian told investigators that after the murder, T.J. Harrelson drove to Gibian’s house in the ex-Met’s Infiniti, prosecutors said.

Harrelson drove his pal to Lake Ronkonkoma, where Gibian tried to throw a black plastic bag with the sword and his clothes in the lake, but it was frozen, according to the videotaped confession.

Harrelson then drove to a strip mall where Gibian threw the bag in a trash bin, he told investigators.

Authorities said Harrelson knew Gibian was trying to get rid of the murder weapon and evidence related to the killing.

“They are both bad seeds,” the victim’s aunt said.

Asked why Harrelson wasn’t facing more severe charges, Collins said, “Harrelson’s involvement was after the fact. He rendered assistance to the defendant after the murder.”

Collins said prosecutors got to Harrelson even before Gibian was formally charged with the killing last year.

“There has been a standard cooperation agreement between this office and T.J. Harrelson and his attorney since March of 2005, prior to the arraignment of Zachary Gibian on murder charges,” Collins said.

In March, a judge ruled that Gibian’s confessions were admissible as evidence in the murder trial, which is set to begin Oct. 4.

Harrelson, whose father is now the co-owner of the Long Island Ducks, a minor league baseball team in Central Islip, could not be reached for comment.

Gibian’s lawyer did not respond to requests for comment. Gibian’s lawyer has claimed the suspect killed his stepdad because he was abusive, a charge that authorities deny.

Prosecutors would not say whether Harrelson could still face jail time.

“Mr. Harrelson’s obligation to the people is to cooperate with law enforcement and provide truthful testimony,” Collins said. “Whether he fulfills his responsibility is a question that will be determined at the trial’s conclusion.”

But Nager’s aunt said Harrelson “definitely has to be held accountable.”

“I want to see justice so badly,” she said. “I want it with all my heart. I didn’t have any children, but if I did, Scott would have been a son. He was killed in his sleep, and then they threw away the bloody clothes.”