Mayor Street and Deputy Police Commissioner Sylvester Johnson sped to the hospital, as other police cars scoured the Fairhill neighborhood looking for the shooter.
Johnson said Volz and his partner were driving south in the 2900 block of Leithgow when another car - with headlights off - came racing toward them the wrong way on the one-way street.
The officers had to pull over to make room for the oncoming speeding car. As the car passed, one of the officers shouted for the driver to turn on his headlights, said Johnson.
Both cars stopped, and one of the men got out, walked over to the unmarked police car and fired one shot, hitting Volz, sitting in the passenger's seat, in the arm. The shooter ran back to the car which sped away.
Volz's partner drove straight to Episcopal. Volz is married with one child.
"The officer was very, very lucky. He could have been killed," Johnson said.
Street described the officer's injury as a "flesh wound."
Volz declined to comment on the incident.
On Sept. 21, Officer Jose M. Ortiz died of injuries suffered four days earlier when another officer, coming to his aid, hit him with a patrol car.