About 9:40 p.m., Ruffin opened fire on his target, a 24-year-old man, and hit him in the head, Clark said. The man was listed in critical condition at the Hospital of the University of Pennsylvania.
Bullets also struck three other people, including Tynirah Borum, 3, who was hit in the chest and died later at Children's Hospital of Philadelphia.
The two other victims - a man and a woman - were wounded, but survived.
A 28-year-old woman was shot once in the left arm and a 21-year-old man once in the right leg. Both were listed as stable at the Hospital of the University of Pennsylvania, Clark said.
Woods, 22, of the 1700 block of Ellsworth Street, was arrested shortly after the shooting and is in custody, facing murder charges.
Clark said Tynirah lived around the corner from the shooting scene and was at the home of her mother's girlfriend having her hair braided on the front steps when she was gunned down.
He said that there was no relationship between Tynirah and the man who was the target and that the child "just happened to be on the street."
Ruffin arrived on the bike and had a brief conversation with the 24-year-old man before he fired the gun several times, Clark said.
Clark said there had been a "back-and-forth beef" between the 24-year-old man and Ruffin and Woods. Clark said police did not know the nature of that disagreement.
He said Woods and Ruffin were "very well-known to police," adding that they had been charged with several violent crimes over the years.
Clark praised neighbors who gave police information identifying Woods and Ruffin.
Inquirer staff writer Robert Moran contributed to this article.