Anthony Comello, 24, was arrested on Saturday in New Jersey over the death of Francesco “Franky Boy” Cali last week in front of his Staten Island home.
While waiting for a court hearing to begin in Toms River, New Jersey, in which he agreed to be extradited to New York, Comello held up his left hand.
On it were scrawled pro-Trump slogans including “MAGA Forever”, an abbreviation of Mr Trump’s campaign slogan “Make America Great Again”. It also read “United We Stand MAGA” and “Patriots In Charge”.
Comello’s lawyer Brian Neary would not discuss the writing on his client’s hand or say whether Comello maintains his innocence.
Asked by reporters after the hearing what was on Comello’s hand, Mr Neary replied: “Handcuffs.”
He referred all other questions to Comello’s Manhattan lawyer Robert Gottlieb, who said his client has been placed in protective custody due to “serious threats” made against him, but gave no details of them.
“Mr Comello’s family and friends simply cannot believe what they have been told,” Mr Gottlieb said. “There is something very wrong here and we will get to the truth about what happened as quickly as possible.”
Comello sat with a slight smile in the jury box of the courtroom on Monday afternoon as dozens of reporters and photographers filed into the room.
When they were in place, he held up his left hand to display the writings as the click and whirr of camera lenses filled the room with sound.
During the hearing, Comello did not speak other than to say “Yes sir” to the judge to several procedural questions.
Cali, 53, was shot last Wednesday by a gunman who may have crashed his truck into Cali’s car to lure him outside. Police said Cali was shot 10 times.
Federal prosecutors referred to Cali in court filings in recent years as the underboss of the Mafia’s Gambino family, once one of the most powerful crime organisations in the country. News accounts since 2015 said he had ascended to the top spot.
Cali’s only mob-related criminal conviction came a decade ago when he pleaded guilty in an extortion scheme involving a failed attempt to build a Nascar track on Staten Island. He was sentenced to 16 months behind bars and was released in 2009.
Police have not said whether they believe Cali’s murder was a mob hit or whether he was killed for some other motive.
The last Mafia boss to be killed in New York City was Gambino don “Big Paul” Castellano, who was assassinated while getting out of a black limousine outside a high-end Manhattan steakhouse in 1985.