The 18-year-old college freshman stabbed to death in Manhattan’s Flatiron District last week was mistaken for a gang member when he was attacked while traveling home for Christmas — which his devastated family won’t be celebrating in the aftermath of the tragedy, according to police and relatives.

Bronx native Denzel Bimpey was returning home for the holidays from SUNY Morrisville, where he was studying business administration in the hopes of one day starting his own company, his older brother told The Post Wednesday afternoon.

“It’s not really a Christmas to celebrate. We are mourning him, so we cannot celebrate Christmas,” the grieving 22-year-old, a recent college graduate who did not want his name published, said at his family’s Morrisania apartment.

“There’s no happiness this Christmas. Christmas is not going to be celebrated this year, it can’t be celebrated this year,” he said.

As he was traveling from Syracuse to New York City, police said Bimpey and his friends got into an argument with another group of college students — one of whom ended up knifing him in the chest.

“While they’re in Syracuse, the two groups are engaged in a verbal dispute, where the perpetrator group starts talking some gang language asking our victim’s group ‘what block you from?,'” NYPD Chief of Detectives Joseph Kenny said Tuesday.

“And they respond, ‘we’re not in a gang.'”

The rest of the bus ride downstate was uneventful, but the spat turned violent when the bus arrived near East 26th Street and Park Avenue South around 10:40 p.m., and passengers went to get their luggage, cops said.

“At this time, our victim attempts to grab the male who ends up stabbing him in a bear hug to keep from getting stabbed,” Kenny recounted to reporters. “Unfortunately, it doesn’t work out, and he gets stabbed three times.”

A wounded Bimpey ran down the block with his friends with the other group — students at Onondaga Community College from Harlem — hot on their trail.

That’s when Bimpey “drops to the ground and starts to bleed out,” Kenny said.

“The perpetrator group realizes he’s down, and they run back up Park Avenue where they enter into a black SUV and flee the scene,” the police official added.

“The victim’s group has no prior arrest history. It just seems to be students that came across a group that may have mistaken them for other gang members.”

The fatal dispute is believed to be tied to a feud between rival gangs, the Six Blocks of Harlem and the Bronx’s Nine Blocks, according to cops.

“We believe our victim group may have been mistaken as Nine Block members,” Kenny said, adding that Bimpey and his friends were not wearing gang colors. 

Detectives were reviewing security footage from the Syracuse bus station that showed the murder suspect and his friends getting out of a car that was traced to Onondaga Community College.

The suspected attackers appeared to have been staying involved with gang beefs on social media even though they attend college hundreds of miles away, officials said.

The NYPD said an unidentified person of interest was in custody Tuesday, but charges had not been filed as of Wednesday, the department confirmed.

The victim’s sister Godslove Bimpey, 28, stressed that her brother was not involved with gangs when reached by The Post Wednesday.

“I can’t comment on the ongoing investigation but my brother was not a part of no gang,” the emotional woman said over the phone.

When asked how the family was holding up while planning Bimpey’s funeral, Godslove said, “not good.”

“We just need the support; It’s not easy to plan a funeral, so we have a GoFundMe account going around, and that’s all I have to say. There’s nothing else I have to say. We just need help.”

Bimpey’s brother added that the senseless attack had cast a dark shadow over the holiday season.

Bimpey attended the NYC Charter High School for Architecture, Engineering & Construction Industries in the Bronx before heading to college, and aimed to “go into the business field” once he completed his degree, his brother said.

The high school held a candlelight vigil for Bimpey in the cafeteria Monday, according to students walking home from class on Wednesday afternoon.

“It was beautiful but very sad,” one girl said of the ceremony. “Everyone was crying, students, teachers … He was loved by many. He’ll be missed.”

Another former classmate lamented Bimpey’s unfulfilled potential and said the late teen was “never the type to be in any gang or anything like that.”

“That shouldn’t have happened to him. He was a good person,” the boy said.

“This is so sad [that he died] for nothing,” the student continued.

“I couldn’t believe it. He just started college. I feel bad.”

A girl who used to eat lunch with Bimpey said she was in shock when she got the news and “couldn’t process it.”

“He would sit with us at lunch last year. He was a very funny person, always joking. When he told a joke everyone laughed. When he’d be absent from lunch we’d wonder if he was okay,” she said.

Another student said Bimpey had always helped him out if he were in a jam.

“I knew him. He was a good kid. He helped me out a lot. If I lost something, he helped me to look for it. He would lend me some cash and I’d pay him back,” the teen said.

“I’m going to miss him.”

SUNY officials said university police were “assisting” the NYPD in the investigation into the homicide of the “first-year business student.”

“Our entire college community is devastated at the loss of one of our students and we are especially heartbroken by the tragic circumstances surrounding Denzel’s passing,” said Theresa Kevorkian, executive director of the Morrisville College Foundation, which gives out scholarships and awards to hundreds of students at the state school.

Bimpey was remembered as a “wholesome spirit” with an infectious grin by his sister during an interview Sunday.

He was excited to see his family after wrapping up his first semester of college, though he had been sick for several days.

“He was your average kid from the Bronx, trying to do better. He had great role models ahead of him,” Godslove said.

“He knew when something was wrong with you, internally, without you even saying a word. He would randomly text me like, ‘Hey, I miss you. I love you,'” she said.

“At this moment, we just don’t understand,” Godslove continued. “We’re hurt. We’re extremely hurt. It did not need to get to this point. These crimes need to stop.”

Bimpey, a devoted student of the game of basketball, also had three brothers and often took care of his ailing mom, who is utterly devastated by his murder, Godslove said.

“Everywhere you turn, you will see my brother’s face,” Bimpey’s grieving sister said of her brother’s killer earlier this week.

“You will hear his name. He will taunt you. Because the God we serve will do that. The God we serve is going to prevail, and we’re going to get justice because we serve a higher power.”

More than a dozen members of Six Block were arrested in 2016 for running an open-air drug market in Washington Heights where they sold $24,000 of oxycodone, cocaine, heroin, Xanax and weed to undercover cops, officials said at the time.

Additional reporting by Steven Vago