Anthony Moore of the 7900-block of South Sawyer was charged with first degree murder for Dennis' death, Chicago Police Superintendent Eddie Johnson said Friday afternoon.
"Every officer will tell you that their greatest fear beyond what could happen to them on the job is when violence they fight so hard to prevent hits home," said Johnson. "Every officer in this department took his murder personally vowed not to stop until we found his killer."
"We promised we'd never stop & we didn't," Chief Spokesman Anthony Guglielmi wrote on Twitter Friday morning.
Arshell "Trey" Dennis III was home from college to surprise his sick mother for her birthday.
Just after midnight on Aug. 14, 2016, police said he was sitting on a porch with a 20-year-old man, who neighbors described as a friend, outside a home in the 2900-block of West 82nd Street. That's when a gunman walked up and opened fire. The victim was shot in the chest. He died at Little Company of Mary Hospital at 12:45 a.m.
"These offenders basically saw three male blacks sitting on a porch and they decided this was the opposing gang and they decided to shoot these individuals," said Commander Brendan Deenihan.
His father, Arshell "Chico" Dennis, is a long-time CPD officer and a close, personal friend of Chicago Superintendent Eddie Johnson. They served together as patrol officers in the 6th District.
"Officer Dennis dedicated his life to make this city safer, and his son Arshell was a good kid, making his parents proud and studying for a promising future as a journalist," Johnson said in a statement.
Dennis III was a graduate of Urban Prep Academy and a junior at St. John's University in New York City. He was slated to go back to school the same day he was killed.
"Our family is deeply saddened by this tragic and senseless shooting," according to a statement from the Dennis family. "The loss of our son is stunning and painful. Trey was smart, funny and the light of our lives."
Brenda, a neighbor who asked to be identified by only her first name, said she was in her living room when she heard several shots outside. She thought they were coming from her porch, so she crawled to the back of the house. After about five minutes of silence, she looked outside and realized the shooting happened across the street.
"I think it was the mother, and she was crying so hard I went to the back of the house to not hear it," she said. "He was a promising child. He was going somewhere-handsome, intelligent and somebody must have mistaken his identity."
Terri Bachstrom, who lives on the same block as the Dennis family, said she knew the victim from working as a Chicago Public Schools lunchroom attendant. She said he was a quiet, well-mannered and great kid.
"He wasn't in a gang. He wasn't affiliated with any of the nonsense that's going on in Chicago," Bachstrom said. "He wasn't one of those kids. He was just a different child all around the board, so I don't understand why somebody would come in and shoot like that."
Moore is expected to appear in bond court Saturday.
The Sun-Times Media Wire contributed to this report.