Federal organized crime experts said yesterday that they believe the murder of Paul (Big Paul) Castellano, who was gunned down Monday night outside a midtown Manhattan steak house, was approved by the Gambino organized crime family that he headed.

Law enforcement sources said that the murder of Castellano, 70, probably would not trigger a gangland war among New York's five major organized crime families. They suggested that any future violence is more likely to be limited to continuing power struggles within each of the families.

Castellano, who headed the nation's largest and most powerful organized crime family, and Thomas (the Wig) Bilotti, 45, his chauffeur and bodyguard, were riddled by bullets fired by three men in trench coats at 5:26 p.m. on a street crowded with commuters and Christmas shoppers.

The bloody bodies lay in the street, covered with sheets, beside Castellano's black Lincoln limousine, for more than two hours as police and FBI agents took photographs and dusted the area for fingerprints. The street was littered with bullet casings, but the weapons, including two semiautomatic handguns, were not recovered.

FBI spokesman Lane Bonner said, "The FBI had received indications of strife within the New York families. The murders are deplorable but predictable in light of the history of organized crime families in using violence to eliminate competition."

Assistant FBI Director John L. Hogan, who heads the bureau's New York operations, said yesterday that it is "too soon to tell" whether the murders of Castellano and Bilotti will lead to further warfare among organized crime figures.

"If it was internal strife within one group and they were cleaning house and have solidified their base, it may be over and done with. If it's outside the group, there could be more action," he said.

Other federal and local sources said that Castellano had long been unpopular with certain factions of the Gambino family. They said his problems escalated just a week before his murder when underboss Aniello (Neal the Lamb) Dellacroce died of cancer.

The sources said that Dellacroce -- who was seen by many as a more powerful figure than Castellano -- had controlled the various factions of the family, often acting as a peacemaker. With Dellacroce dead, they said, his backers feared they might be cut out of the action by Castellano.

In addition, Bilotti was in line to become the new underboss, a move that was resented by Dellacroce loyalists.

Ronald Goldstock, executive director of the New York Organized Crime Commission, also pointed out that Castellano had become a liability because of the number of legal proceedings in which he was involved.

At the time of his death, Castellano was a defendant in one of a series of simultaneous trials of organized crime figures going on in New York's federal courthouse in Foley Square. The case against Castellano included a 78-count indictment on charges including car theft, extortion, narcotics trafficking and murder.

Castellano was also scheduled to go on trial March 17 with the leaders of the four other New York organized crime families on charges that they participated in a "commission" that controlled mob activities ranging from murder and drug trafficking to loan sharking, gambling and labor racketeering.

"If you take a look at his family and the other families in New York and polled them as to whether he was an asset or a liability, everyone would say he was more of a liability and that they would be better off with him dead . . . . It's very difficult for the other members of the family to take advantage of criminal opportunities when their chief officer is tied up in court," Goldstock said.

Goldstock said there was considerable fear among organized crime figures that Castellano might cooperate with investigators. "There were rumors that Dellacroce had cooperated, and people thought that if the underboss can cooperate, then why can't the boss," Goldstock said.

He said that if Castellano's co-defendants had been asked "to raise their hands if they wanted him alive, there wouldn't be too many raised . . . . As long as everybody felt they would be better off with him dead, there was no one to protect him."

The New York trials are the culmination of a huge federal and state assault on organized crime's most powerful base. According to the FBI, nearly half of the nation's 2,000 organized crime members and their 20,000 associates live in the New York area. About 300 suspects are under indictment in federal and state organized crime cases in New York.