Testimony in the trial of a white former South Carolina police officer in the shooting death of an unarmed black motorist is raising the questions about how much force is justified.

Michael Slager faces 30 years to life in prison if convicted of murder in the April 2015 shooting death of 50-year-old Walter Scott, who was shot and killed fleeing a traffic stop.

The prosecution's case enters a fourth day in Charleston on Tuesday.

Scott was shot in the back in a shooting captured on a cellphone video that stunned the nation. He ran after being pulled over for a broken taillight on his car.

On Monday, police officers testified there are various levels of force officers can use to get suspects to comply. The lowest level being just their presence wearing a uniform which can be followed by voice commands, use of force and eventually deadly force.

North Charleston police Sgt. James Gann, who responded to the scene following the shooting, testified shooting someone in the back is not part of any officer's training. He said the video of Scott running away from Slager indicated to him that Scott was trying to ease the level of confrontation.

"Through the video I would agree that he would be deescalating," a situation in which an officer would have been taught then to use less force, Gann testified.

Responding to questions from Slager's attorney Andy Savage, he agreed Slager had no way to know if Scott was armed and was running to get behind a tree and fire back.

He also testified Scott showed no sign he was surrendering and that there was no way Slager, in a few split seconds, could know Scott's intentions.

Gann also testified Slager was no hothead and had "always been even keeled."

Meanwhile, law enforcement officials are investigating nine suspicious letters mentioning racial violence sent to hotels, a church and a park in Charleston amid the Slager trial and another across the street in federal court with racial overtones.

Charleston city officials say local police and the FBI are investigating letters sent last month from outside the United States. They were received by hotels, Emanuel AME Church and a local park where a popular holiday light display opens this week.

Jury selection resumes Wednesday in federal court in the trial of Dylann Roof who is charged with hate crimes, obstruction of religion and other counts in the shootings of nine black parishioners at Emanuel in June of 2015.