The 16-year-old Brooklyn boy accused of setting the mattress fire that killed an NYPD officer last month pleaded not guilty Thursday at his arraignment to felony murder and arson charges, officials said.

Marcell Dockery entered his plea in State Supreme Court in Brooklyn as an estimated 70 police officers looked on in the courtroom.

He was indicted last month for murder in the second degree after allegedly starting the smokey fire shortly after noon on April 6 in a Brooklyn public housing building at 2007 Surf Avenue.

Officer Dennis Guerra, 38, and his partner, Rosa Rodriquez, 36, responded to the alarm by taking the elevator to the 13th floor where they were quickly overcome by the smoke and lost consciousness.

Guerra died April 9 from the effects of smoke inhalation and carbon monoxide poisoning. Rodriquez remained hospitalized Thursday in critical but stable condition. In the aftermath of the blaze, the NYPD instructed officers to take the stairs when responding to high rise fires.

Dockery was originally charged with arson after police said he told them he set the fire because he was bored.

When Guerra died the case was upgraded to felony murder by the Brooklyn District Attorneys Office. Dockery remained in custody without bail after his court appearance.

His attorney, Jesse Young, couldn't be reached for comment late Thursday. In the past Young has insisted that Dockery was innocent.

Outside the courtroom, Patrolmen's Benevolent Association President Patrick Lynch said that despite his youth, Dockery mus be held accountable.

"Don't be fooled by the young face. His actions were serious. His actions were adult actions. His actions were life and death," said Lynch. "He said he did this because he was bored. Well he will have to get used to boredom in that prison cell."