A former judicial aide and grandson of a longtime North Philadelphia state legislator was sentenced to three to six years in prison Tuesday for his role in a series of 2015 armed robberies that left four people shot and seriously wounded.

Frank Oliver III, 27, was the getaway driver for Amin "Mac" Ackridge, dubbed the "ruthless robber" by authorities for gratuitously shooting his victims after getting their valuables.

Ackridge, 31, of Strawberry Mansion, was found guilty April 28 of attempted murder, conspiracy, and multiple counts of assault, robbery, and gun crimes. He will be sentenced July 17.

Oliver, who testified for the prosecution at Ackridge's trial, apologized to his family and to Philadelphia Common Pleas Court Judge Jeffrey P. Minehart for his conduct.

Oliver's grandfather Frank L. Oliver, now 95, a North Philadelphia Democratic ward leader who served in the state House from 1973 to 2011, was in court but did not speak.

In addition to prison, Minehart sentenced Oliver to 15 years' probation after his release.

Defense attorney Fortunato N. Perri Jr. said the sentence recognized Oliver's cooperation with the prosecution and his testimony before the grand jury and at Ackridge's trial.

Assistant District Attorney Jason Grenell said his office did not recommend a sentence. In addition to his testimony, Grenell added, Oliver has been in prison outside Philadelphia because of threats on his life.

Before his arrest, Oliver had worked as a personal assistant to Common Pleas Court Judge Roger F. Gordon. He was a court employee from July 14, 2014, until May 28, 2015.

Authorities said Ackridge and Oliver were responsible for four robberies in June and July 2015, three in Kensington and one in North Philadelphia.

In each, a car approached and pulled ahead of the victim. A gunman got out of the car, then robbed and shot the victim before fleeing in the waiting car.

In late July 2015, Oliver was driving in Center City when he was stopped by police.

Under Oliver's car seat was a 9mm semiautomatic pistol for which he did not have a license. Police ballistics technicians checked and found the 9mm matched the ejected cartridge casings found at the scenes of the robberies.

At first, police detectives thought Oliver was the shooter, but victims and witnesses said Oliver was too heavy to have been the gunman.

Detectives then began looking at Oliver's cellphone records and noticed that on the days of the robberies, he called Ackridge, and that their cellphones often pinged off cell towers near the robbery scenes.

Ackridge was arrested Aug. 12, 2015, in Wilmington as he allegedly was about to commit an armed robbery.