Marcell Dockery, 16, is walked at the 60th Precinct on...

Marcell Dockery, 16, is walked at the 60th Precinct on West 8th Street in Brooklyn on Monday, April 7, 2014, after he was arrested for his alleged involvement with a fire in Brooklyn that resulted in critical injuries to two NYPD officers. Credit: Theodore Parisienne

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 on 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.

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

Officer Dennis Guerra, 38, and his partner, Rosa Rodriguez, 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. Rodriguez 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 attorney's office. Dockery, who faces a maximum of 25 years to life in prison if convicted, remains in custody without bail.

His attorney, Jesse Young, couldn't be reached to comment. Young has insisted that Dockery is innocent.

Outside the courtroom, Patrolmen's Benevolent Association President Patrick Lynch said that despite his youth, Dockery must 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," Lynch said. "He said he did this because he was bored. Well, he will have to get used to boredom in that prison cell."

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