Just weeks from entering his junior year at St John's, the aspiring journalist and NAACP student chapter vice-president flew back home to Chicago for the weekend, WGN TV reported, a surprise for his sick mother's birthday.
He sat on the porch of his family's home at the weekend, talking to a hometown friend, when gunfire split the stillness of his ordinarily quiet Wrightwood neighbourhood.
Both were shot.
The friend, 20 years old, was hospitalised.
Arshell died.
His mother's screams echoed down the block.
"You do not want to hear a mother's cry for her son," a neighbour, who would only identify herself as Brenda, told the Chicago Tribune.
As of yesterday, there had been no arrest, and authorities said the investigation was still open and active.
In a press conference, Chicago Police Department Superintendent Eddie Johnson said authorities were treating the shooting as a case of "mistaken identity".
"Arshell was a good kid, making his parents proud and studying for a promising future," Johnson said.
The superintendent, who worked with Arshell's father, Officer Arshell "Chico" Dennis, in the 1990s, visited the family and said in the statement he was "at a loss for words for the amount of grief" they are experiencing.
Though it's well-known in the neighbourhood that the elder Dennis, a DEA task force officer, worked for the Chicago Police Department, the superintendent said there is "absolutely no credibility" to the theory that Arshell was targeted because of his father's occupation.
The two young men had no criminal records or personal histories of gang involvement, a police spokesman told the Chicago Tribune, but the shooting could still be gang-related. Some gangs have been conducting initiations, a police official told the Chicago Sun-Times, where recruits are instructed to shoot and kill whomever they find.
"That's a rite of passage for them," Johnson told CBS News. "Now how bizarre is that?"
On Monday, the day Arshell was to return to New York, his loved ones scrubbed his blood from their pavement instead.
"The loss of our son is stunning and painful," the Dennis family said in a statement to the Sun-Times. "Tragically, we were going to take him to the airport today to return to school. Now because of this senseless violence, we will be grieving and planning his funeral. Trey was smart, funny, and the light of our lives."
Arshell graduated from Urban Prep Academy in 2014, where he ran cross country, played chess and participated in the Louder Than a Bomb poetry competition, according to his LinkedIn page. He also belonged to a preparatory programme called Upward Bound. It was from a student in the programme that director Gerald Smith heard the tragic news.
"I got the phone call, and my heart just fell to my stomach," Smith told the Tribune. "So, so unexpected. . . . I'm still in disbelief."
Last summer, Arshell returned to Chicago to work as an Upward Bound ambassador, Smith said.
"He was one of my better students, he really was," Smith said. "Arshell was a fun time. He was real easygoing, real quiet, laid back, mild-mannered - he wasn't a problem at all. It's a tragic loss."
Johnson said at the press conference that to date in 2016, 85 per cent of gunshot victims have had prior contact with police. Arshell and his friend were the exception, and yet this weekend, the two became part of the city's growing violence statistics. In the same weekend Arshell died, nine people were killed and 31 more wounded in shootings across Chicago.
Since January 1, 2607 people have been shot in the city, according to a Chicago Tribune analysis, on pace to far exceed the number of shootings last year, which totaled 2988.
