Mob guys have been killing each other since they began taking blood oaths to Cosa Nostra a hundred years ago. But it’s not every day – or every year, for that matter – that they whack a director of real estate for the city’s Marine and Aviation Department.

The rubout of Rick Mazzeo, who was shot four times in the head, wrapped in a plastic bag and placed in the trunk of his rented car – and then stabbed seven times through the plastic bag after he was dead – has remained unsolved for 20 years.

The execution – Mazzeo, 35, was killed Nov. 14, 1983; his body was recovered five days later – had been doubly intriguing because at the time he was closely linked to Roy Cohn, the late power broker and political fixer whose activities with Mazzeo were then under investigation by the feds.

Mazzeo, whose city salary was $15,000 a year, became a millionaire dispensing lucrative leases for newsstands and parking lots for Staten Island ferry terminals on both sides of the harbor. He drove a Mercedes with a vanity plate, “Gatzby,” and rubbed elbows with lawyers, judges and elected officials at Cohn’s garish birthday parties at Studio 54.

From the witness stand at the racketeering trial of Bonanno boss Joseph Massino, family turncoat Richard (Shellackhead) Cantarella (right) has cleared up the

mystery. Massino (left) is charged with ordering seven murders between 1981 and 1987, but not Mazzeo’s.

“Who wanted him killed?” assistant U.S. attorney Mitra Hormozi asked Cantarella.

“Me,” said Cantarella, naming capo Al (Al Walker) Embarrato and soldier Joseph D’Amico – Cantarella’s uncle and cousin respectively – and mob associate Patrick (Patty Muscles) Romanello as his more than capable assistants in the execution.

Like many of Cantarella’s gangster activities, Mazzeo’s slaying was put in motion at The New York Post, where Cantarella, now 60, began working in 1963 with a “walking route” under tutelage from Uncle Al Walker, who died three years ago at age 92.

While engaging in mob staples like extortion, bookmaking and loansharking – at The Post and elsewhere – Shellackhead drove a newspaper delivery truck, allegedly. By 1992, when he was nailed on state labor racketeering charges, Cantarella had worked his way up to a full-time $41,600-a-year no-show job that included health and welfare benefits.

His no-show job was “riding the tail” of the truck. Instead, he was “selling the tail,” he testified. “Instead of showing up, I hired a guy for $20 a night to do my job...My salary was $800 a week gross.”

He also got into newsstands and parking lots, he testified, first by squealing to

Mazzeo that a newsstand operator was selling football tickets so that the bribe-hungry Mazzeo would have an excuse to break the then-proprietor’s lease.

“I bribed him every month,” he said, estimating that he paid Mazzeo “a couple hundred thousand” from the mid 1970s to 1980.

By the fall of 1983, however, Mazzeo had become a liability.

Fired by the city, Mazzeo had been convicted of tax charges and served six months in prison. “He got thrown out of his job (and) he was taking drugs,” said Cantarella. “We were concerned he was going to cooperate. Me and my uncle was concerned.”

Turncoat wiseguy Joey D’Amico, one of many Bonanno mobsters and associates on The Post payroll, picks up the story.

“On the afternoon of the murder,” D’Amico told the feds, “Embarrato and Cantarella met (me) near the New York Post building … and asked (me) to help kill Mazzeo.” They also told D’Amico to enlist additional help from Romanello.

That night, D’Amico picked up Patty Muscles and hooked up with Cantarella, Embarrato (left) and Mazzeo, who were already at a private sanitation

garage in Brooklyn’s Bushwick section that had been selected for the hit.

At the garage, Al Walker stood guard outside, while D’amico and Patty Muscles waited in a truck bay. Cantarella walked upstairs to the office where Mazzeo, who thought he was there to see about getting a job, was talking to the owner.

“I met Rick in (the) office.” Cantarella testified. “I walked him to the garage area. I asked him what kind of car he was driving, he told me and I said, ‘Let’s take a look.’ While he was walking down the stairs I shot him in the back of the head. He was laying on the floor all bloody.”

At that point, according to D’Amico, Patty Muscles, (right) a sentimental kind of guy, suggested that they should each shoot Mazzeo one more time “for camaraderie.” So D’Amico, Cantarella and Romanello all did.

They wrapped Mazzeo in black plastic bags and carried him out to his car. As they put his lifeless corpse in the trunk, Uncle Al Walker interjected, “Make sure he’s dead.” So, D’Amico told the feds, he “stabbed Mazzeo’s body while the body was in the trunk.”

In January 2003, a few days after Gang Land disclosed that Cantarella was cooperating, sources say a very worried D’Amico visited Romanello and showed him a copy of the column that ran in The New York Sun on Jan. 9 and told him, “We got a big problem.”

Sources said Patty Muscles recalled that when his father died the previous summer, Shellackhead had sent flowers to his father’s wake and expressed some concern that he knew his real name. In the end, according to one source, Romanello told D’Amico “not to worry. If they didn’t say a word, they would be all right.”

Cantarella got work at the New York Post for a bunch of relatives, including cousin Frank Cantarella and nephew Joe Padavano. He also got his son Paul (left) put on a shape up list, a precursor to a steady job, “but he got thrown off,” Cantarella testified.

Paul couldn’t make the grade at The Post, but with his dad’s help, he passed muster with the Bonannos and was inducted into the family in a ceremony at his father’s Staten Island home in 1996, according to FBI documents.

This week, however, Shellackhead couldn’t bring himself to testify about his son’s life of crime. He knew nothing about his son’s criminal activities, he said.

Under questioning by Massino’s lawyer David Breitbart, Cantarella – his son is also cooperating with the feds but he is not scheduled to testify at Massino’s trial – went so far as to deny knowing what his son was charged with, even though both were indicted in the same racketeering case in October 2002.

Cantarella said that he hadn’t spoken to his son in 18 months, or any other Bonanno defectors since he began cooperating, thwarting Breitbart’s assertion that they had compared notes to get their stories straight to help the feds convict Massino of crimes that they had committed.

He conceded however that he had learned details about the Massino case, including the names of other wiseguy defectors from his wife, Lauretta Castelli, (right) who was indicted along with her hubby and son and is also cooperating. His wife, Cantarella said, had read newspaper stories about the case and had kept him posted on developments during weekly jailhouse visits with him.

Cantarella also claimed ignorance of another important matter: the five guns investigators seized from his home in 1992. Breitbart introduced records showing the weapons included a sawed-off shotgun, a street-sweeper, a Derringer, and a Beretta. “I don’t remember,” said Shellackhead.