City police are still investigating a weekend homicide in South Philadelphia and also have concluded their probe of a fatal shooting in a Southwest Philadelphia neighborhood that they found was in self-defense.

In the Southwest Philadelphia killing, a man in his 20s was fatally shot twice in the chest shortly after 1 a.m. yesterday in the 5100 block of Warrington Street near South 52d Street. Police spokeswoman Christine O'Brien said the man and a 17-year-old male accomplice were trying to rob two other men on the street, who shot them, O'Brien said.

The teen was shot twice in the left arm and was in custody in stable condition at the Hospital of the University of Pennsylvania last night.

Police did not release the identities of the two would-be robbers or of the pair they were trying to rob.

O'Brien called the shooting "a justifiable homicide" and said no charges would be filed.

The other fatality occurred after 10:30 p.m. Saturday in a rowhouse in the 2600 block of Wilder Street in South Philadelphia. Anthony Williams, 37, who lived there, was struck in the head with an object and died en route to the hospital, O'Brien said.

Police arrested Raymond Armstrong, 31, of the 500 block of Allen Street in Allentown, in connection with Williams' death.

O'Brien said an argument apparently sparked the attack.

Contact staff writer Jan Hefler at 856-779-3224 or jhefler@phillynews.com.