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When a hooded gunman pumped close to a dozen bullets into Frank Cali, a reputed leader of the Gambino crime family, outside his Staten Island home on Wednesday night, detectives naturally examined the possibility that the killing may have stemmed from his role atop one of New York’s five Mafia families.

But after the arrest on Saturday of a 24-year-old man who lives with his parents, it appears the old-school crime figure may have been the victim of an angry young man with a gun and a grudge, but apparently no mob ties.

Some law enforcement officials have said they are looking at the possibility that the alleged killer, Anthony A. Comello, had a romantic interest in one of Mr. Cali’s female relatives, and that the crime boss had told him to stay away. But the Police Department’s chief of detectives, Dermot F. Shea, said the motive was still unclear, and detectives were examining “multiple angles.”

Mr. Comello’s arrest was in some ways a sign of the times in an era when the city’s mob clans have become accustomed to a smaller, less powerful role in the criminal and economic culture. Weakened over several decades by a campaign of prosecutions, the Mafia has become less likely to resort to violence, even as gun violence and mass shootings have become a common occurrence in the United States.