By GREG B. SMITH DAILY NEWS STAFF WRITER

The name of a second detective surfaced yesterday during a trial involving allegations that a handful of Brooklyn cops have been moonlighting for the mob. Prosecutors said that Detective Michael Pierre was part of the coverup of the messy murder of a longtime gangster. He has since agreed to be a prosecution witness, they said.

His name was revealed during the opening of a mob trial in Manhattan Federal Court in which Detective Michael Silvestri figures.

Pierre was Silvestri's partner in Brooklyn's 76th Precinct in 1998, when the FBI says they tried to cover up the slaying of Joseph (Joe Pitts) Conigliaro.

Yesterday, Police Commissioner Raymond Kelly played down the mob scandal, insisting that only Silvestri and Pierre were involved.

"There are two officers who are involved in this investigation," Kelly said. "There are no other aspects of this investigation involving members of this department."

But as Kelly spoke to reporters, prosecutors a block away played a tape of an informer named Ralph talking about a third, unnamed, cop "doing favors for us."

A fourth cop has been named by another informer as hiring a gangster in an arson plot. The allegation surfaced in FBI documents related to the investigation of the DeCavalcante crime family that ensnared Silvestri.

Silvestri and four low-level gangsters are charged in the killing of Conigliaro, a capo in a wheelchair whose own crew turned on him.

Prosecutors say Silvestri removed bullet casings from Conigliaro's car for his cousin and co-defendant, Joseph (Big Joey) Brideson, a Mafia associate who allegedly set up Conigliaro for murder.

They also allege that Silvestri traced the license plates of loanshark victims and tipped off gangsters when their names surfaced in another murder case.

Yesterday, Assistant U.S. Attorney Dani James painted Silvestri as a corrupt cop who chose the wrong path the night of the alleged coverup.

'Turned his back'

"On that night, [Silvestri] had a choice - to be an honest cop or to do what the Mafia asked him to do," she said. "Detective Silvestri turned his back on his fellow officers and the law."

But Silvestri's attorney, Stephen Brounstein, portrayed the detective as a hard worker with multiple commendations who was being set up by his big-mouth mobster cousin.

"He is not here because of what he did. He is here because of what somebody said he did," Brounstein said.

James also named Silvestri's partner, Pierre, as an accomplice in covering up the Conigliaro hit.

"It was to Pierre that Michael Silvestri turned when he learned he was jammed up," James said.

Later, the prosecutor said, Pierre changed his mind about helping Silvestri and "decided to break what is often referred to as the Blue Wall of Silence."

Pierre, who has agreed to testify against his partner, could not be reached for comment yesterday. NYPD officials would say only that Pierre was on desk duty.