(CNN) -- New York police are searching for a man who stabbed two children, one fatally, in an elevator Sunday evening.
Police described the man as a "heavy set" black male, approximately 6 feet tall, between ages 25 and 35, and last seen wearing a gray shirt. He stabbed a 6-year-old boy and a 7-year-old girl inside an elevator in their apartment building in the East New York neighborhood of Brooklyn and fled, police said.
Prince Joshua "PJ" Avitto, 6, was stabbed in the torso and transported to a nearby hospital, where he was pronounced dead. The girl was also stabbed in the torso and remains in critical condition, according to police.
New York Police Commissioner Bill Bratton said Monday that the two children were on their way to get ice cream when they were stabbed.
"The amount of violence we saw in an unusual weekend reinforces the need for an energized police department," Bratton said. "We cannot have a city where two ... children going to get an ice cream are stabbed, one killed and the other fighting for her life. That is not the City of New York that we want to police."
Families of both children have been coping with grief and have struggled to comprehend the situation.
"It's two children. It doesn't make any sense," said Scott Avitto, PJ's uncle. "We're all just struggling, we're trying to pull it together. Our family is strong, but we just want justice to be done."
"Pray for her, and us. And pray for PJ's family. This is senseless. I don't know what monster would do something like this," Regenia Trevathan, the girl's great-grandmother, told CNN affiliate WCBS. She said adults were watching the children outside on a playground when they went inside the building alone to get ice cream.
Avitto was related to Taj Gibson, a forward for the Chicago Bulls. Gibson tweeted Monday morning: "They killed my lil super man. #rippj only two more weeks until your 7 birthday. Tears forever." Gibson also posted a picture of Avitto to his Instagram account.
PJ's godfather Henry Alston said the boy's funeral will be Friday.
Police have recovered a Dura Edge knife that was used in the homicide.
Investigators are considering the possibility that the suspect may also be responsible for another fatal stabbing that occurred just blocks away.
Tanaya Copeland, 18, was found in the same neighborhood Friday night, unconscious and unresponsive with multiple stab wounds to her torso. Copeland was pronounced dead at a nearby hospital, police said.
The close proximity of the incidents, as well as similarities in the type of weapons used, have led detectives to believe the two cases may be related. No arrests have yet been made in either case, and the investigation is ongoing, police said.
CNN's Adam Reiss contributed to this report.