A teenager was convicted Thursday of setting a mattress fire that killed a New York City police officer and severely injured his partner after the two were overcome by smoke and carbon monoxide.

Marcell Dockery, 18, was convicted of murder, arson and assault charges in connection with the April 2014 fire that killed Officer Dennis Guerra and injured his partner, Rosa Rodriguez.

Prosecutors alleged Dockery set fire to a mattress in the hallway of a public housing apartment building in Coney Island on April 6, 2014. Guerra and Rodriguez were responding to the scene when they were overcome by smoke and carbon monoxide. Firefighters found the two officers unconscious. Guerra died three days later and Rodriguez spent six weeks in a hospital.

After the fire, Dockery, who lived in the building, told detectives he used a lighter to set the mattress on fire. A videotape of Dockery's interrogation was admitted as evidence during the trial.

"I decided to take a lighter and light the top of the mattress because I was bored," he told police.

But Dockery, who testified in his own defense, said he lied to the detectives and claimed he discovered the fire and tried to save other residents. He said police threatened to evict his family if he didn't tell the truth and that detectives told him he wouldn't be charged if he confessed.

Dockery's attorney, Jesse Young, said his client was a "patsy" and maintained his innocence. He said police did not find Dockery's fingerprints or DNA connecting him to the mattress and they found no accelerants on his clothes.

"Police Officers Guerra and Rodriguez showed great courage when they rushed into that building to save the lives of others," said Brooklyn District Attorney Kenneth Thompson. "Tragically, Officer Guerra lost his life and his partner suffered severe and permanent life-threatening injuries -- all because the defendant was bored and set a mattress on fire, and did so despite being clearly warned about the dangers of setting fires."

Jurors deliberated for nearly four hours Thursday at state Supreme Court in Brooklyn. In their note announcing they had reached a verdict, the jurors said they believed Dockery "didn't mean to harm anyone" and that they "hope leniency can be shown."

Patrick Lynch, president of the city's rank-and-file police union, the Patrolmen's Benevolent Association, said jurors reached a "difficult but just decision." He said Guerra's family was satisfied with the verdict, but that it wouldn't bring their loved one home.

"So this doesn't fix everything," he said. "But it does bring justice and it does send a message that you cannot kill a New York City police officer no matter how that's done."

Dockery faces up to life in prison when he is sentenced in June.