CHARLESTON, S.C. — A man riding with Walter Scott when they were pulled over by Michael Slager testified Thursday that he doesn’t know why his friend tried to run away before the police officer shot him in the back.

‘‘That’s a question I would like to ask him. Unfortunately I can’t. He was murdered,’’ said Pierre Fulton, who worked with Scott at a distribution warehouse.

Fulton was riding in the front passenger seat when the North Charleston officer stopped Scott over a non-functioning tail light in the used Mercedes he had just picked up.

Fulton said Scott gave Slager his license and stepped out of the car, but was told to get back in again, and complied. Then, as the officer was checking the information, Scott bolted.

‘‘The next thing you know he was out the door,’’ Fulton said.

Fulton testified that he heard gunshots a short time later. Prosecutors said Slager fired eight times as Scott ran away after failing to subdue him with a stun gun, killing the motorist in an unjustified shooting.

A bystander pulled out a cellphone and recorded it, stunning the nation as the images spread on social media. Slager, who is white, faces 30 years to life if convicted of murdering Scott, who was black. He also faces separate federal civil rights charges.

The bystander’s video showed Scott, 50, being felled from a distance after five of the eight shots hit him in the back and buttocks. It did not show the entirety of the encounter; nor did a dash cam video from Slager’s cruiser, which recorded only the traffic stop itself.

Jurors watched that dash-cam video in court on Thursday, which shows Scott suddenly running from his car after Slager goes to his cruiser to check Scott’s driver’s license.