(CNN) The 18-year-old accused of stabbing a high school student to death in a fight that was recorded on bystanders' phones pleaded not guilty Thursday, and his attorney said there is more to the story than what's depicted in the videos.

Tyler Flach is charged with second-degree murder in the death of 16-year-old Khaseen Morris, killed after a fight at a strip mall near Oceanside High School on Monday in Nassau County, New York.

Flach was remanded into custody and is due back in court Monday, according to Brendan Brosh, a spokesman for the Nassau County District Attorney.

Defense attorney Edward Sapone said Flach maintains his innocence.

When the fight broke out Monday, a group of 50-70 teenagers looked on as Morris was assaulted and stabbed in the chest, according to Detective Lt. Stephen Fitzpatrick of the Nassau County Police Department.

Khaseen Morris

Some took out their phones and documented the fight on social media, but no one stepped in to help Khaseen, who died from his wounds after being taken to the hospital, Fitzpatrick said.

At a news conference on Tuesday, before any arrests had been made, Fitzpatrick said it was a "prearranged" fight over a "girl and a perception of who she might be dating or who she might be hanging with at this time."

"That dispute boiled over here, where a group of six to seven males charged at the victim and his couple of friends, and during that melee, stabbed him," Fitzpatrick said, standing in front of the strip mall where the fight took place.

He said several of those in the crowd have come forward with information.

Sapone said Thursday that "there is more to this than what is depicted on the videos" and what some of the witnesses are saying.

"We are not making direct comments concerning the victim at this time," Sapone said. "Instead, we choose to remain completely respectful. However, there is ample evidence that is being discovered by our team that that group has a connection to the Bloods street gang."

Reached by CNN Thursday, Khaseen's older sister, Keyanna Morris, said she was sad and angry to hear Sapone's comments.

"I was disgusted and sick to my stomach that this lawyer tried to paint a picture of my brother that this was gang-related," Morris said.

"He was so against gang violence, so against violence towards anyone," she said.

Khaseen was happy, his sister says

Morris described her brother as "good pure soul" who would walk into a room "just smiling" and make "everyone so happy."

The youngest of four siblings, Morris liked skateboarding and dying his hair, his sister told CNN.

Morris had just started his senior year at Oceanside, where he had made new friends after attending school in Freeport, a village on Long Island, where his sister said he'd been unhappy.

"This is the happiest I've been in years," he told his mother on Friday, according to his sister.

Keyanna, through tears, described watching video of her brother's death.

"He just laid there in his blood and nobody helped him," she said.

The steadiness of the video, focused on her dying brother, pained her.

"That's just sick," she said. "Who would just stand there and watch a kid in blood?"

Fitzpatrick said he didn't know what to make of the number of bystanders who failed to assist Morris. "Kids stood here and didn't help Khaseen. They would rather video this event. They videoed his death instead of helping him," he said.

Memorial next week

Memorial services for Morris will be held on September 28, according to his sister.

GoFundMe account has been set to assist the family with funeral expenses.

Oceanside School District Superintendent Phyllis Harrington said in a video posted Thursday that the district is doing "everything necessary" to protect students and staff. She asked that parents talk to their children about social media use, cautioning users not to "engage in spreading rumors." CNN has reached out to the school district for comment.

Nassau County Executive Laura Curran said in a statement that additional police officers would be assigned to patrol Oceanside High School and neighboring schools. "I want to plead with all our residents -- not just our young people: If you see someone in serious danger, please use your phone to get help -- not likes and shares," she said.