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LOWER MANHATTAN—Inside an elevator at the Boulevard Houses in East New York, a madman with a knife fatally stabbed P.J. Avitto, 6, police said.

His friend Mikayla Capers, 7, was stabbed 16 times but survived.

There were no cameras inside that elevator to capture the vicious crime.

Detective Jason Harvey, of the NYPD Artist Unit, draws a composite sketch.

Det. Jason Harvey, of the NYPD Artist Unit, depended on the brave 7-year-old’s memory to draw a composite sketch of the suspect.

“When I saw that picture, it dawned on me. That guy is a homeless guy. People in my building used to give him meals,” Nicholas Avitto, P.J.’s father said.

Composite sketch of Daniel St. Hubert, charged with stabbing two children in an elevator at the Boulevard Houses in 2014.

The composite sketch and DNA from the knife he used lead police to the suspect Daniel St. Hubert.

For the officers in the NYPD Artist Unit, sketching the likeness of a suspect for a wanted poster is the easy part.

“The moment the victim walks into the room, you have to develop a relationship with that victim. It’s so important, it’s the most important aspect I believe other than the sketch itself,” Detective Tara Benowitz said.

The victims and witnesses that walk through the doors of the three-person Artist Unit, located within on the 5th floor of 1 Police Plaza, are often victims of the most heinous crimes.

The unit cranks out hundreds of drawings each year. Wanted posters bearing their composite sketches have proved critical for detectives.

Benowitz, is the Police Department’s first female sketch artist in the unit’s history.

Her first big case, was sketching the likeness of a repeat predator who impersonated a cop to sexually abuse a 15-year-old boy in Brooklyn.

“The little boy was able to identify him because he had very close contact with him,” Benowitz said.

Composite sketch of Steven Papas, who impersonated a cop to sexually abuse a 15-year-old boy in Brooklyn.

“He was identified from the sketch. The sketch was a very good likeness,” said Former Police Commissioner Ray Kelly, when the suspect was arrested.

The artists use Adobe Photoshop to enhance photos when criminals try to disguise their appearance.

“Detectives will say can you take the beard off, can you take the mustache off. We’ll add hats, add glasses,” Benowitz said.

Harvey often works with the Police Department forensic anthropologist, using high tech tools to put a name and face to skeletal remains.

“I’ll go down and take a look at the real skull and take that real skull and put it in the 3D printer. [The forensic pathologist will] give me that copy and I ’ll take it back here and I ’ll reconstruct it,” Harvey said.

Robert Boyce, the Police Department’s chief of detectives, has pushed for the continued use of sketches.

That move comes at a time, when police departments across the country have relied on part-time contractors and computer programs to build sketches.

“We do something for our victims and provide for our victims that computers can’t. We have a human describing somebody to us. A computer can’t do that.,” Benowitz said.