Bullets flew through the cruiser's windshield, 13 in all. The suspect was shot multiple times, and officers slapped handcuffs on him, Belviso said.
Campbell's body remained on the ground next to the bullet-riddled cruiser for more than five hours after the shooting before it was placed in a coroner's van and taken away.
Markeisha Marshall, a spokeswoman for Walgreens, said the company was "deeply regretful" over the officer's death and extended its sympathies to his family and friends. The store has round-the-clock armed security, Marshall noted.
Police are also searching for another man who they believe was involved in the previous homicide with Campbell, Fulop said. They have been aggressively seeking Daniel Wilson for the last three days, Fulop said.
The Jersey City Police Benevolent Association said in a statement that their hearts were heavy over Santiago's death.
"Patrolman Santiago knew the risks associated with this job, yet he put himself in front of danger in order to keep Jersey City safe," the association said. "Words cannot adequately express our feelings about this senseless tragedy."
The officer's stepfather, Alex McBride, said Santiago was "very proud" to be a police officer, following in the footsteps of his uncle. McBride said he had been in Santiago's life for 14 years, noting that his stepson had wanted to be a police officer since playing the "Call of Duty" video game.
"Melvin was the best kid," he said, choking up as he sat hunched over on a plastic crate in an alley outside the family's apartment. "I watched him graduate from high school. He joined every sport, everything. He never did no harm to nobody. And he was full of life."
Gary Nahrwold, 24, recalled his friend Santiago first saying a decade ago that he wanted to become a police officer. Nahrwold also hopes to join the force and said he won't be discouraged by Santiago's slaying.
"It just gives me more purpose to do it," he said. "I'm not going to be deterred by some senseless crimes."
Associated Press writers Julio Cortez in Jersey City and Ashley Thomas in Philadelphia contributed to this report.