A Taser, which can send an electrical jolt four times hotter than the sun, could have burned North Charleston patrolman Michael Slager’s shirt, or was it a meteor?

Gunshot residue on Walter Scott’s right hand could indicate that he grabbed Slager’s gun before the policeman shot him to death, or did it get there when the officer handcuffed him after the shooting?

Those were the kind of questions posed in front of jurors who on Monday, the 14th day of Slager's murder trial, started seeing evidence that his defense team says portrays the shooting as justified. The key piece: Two defense witnesses — a state investigator and a private expert — said they knew of nothing but a Taser that could have caused the burn marks on Slager's uniform.

Slager’s lawyers have said that Scott got control of the stun gun during an April 2015 confrontation and tried to use it. Slager, fearing Scott would then get his pistol, said he acted in self-defense. The officer shot at Scott eight times as Scott ran away, a scene captured on the video that led to his arrest.

What the video did not catch during the struggle between the men is the core of Slager's defense. The defense’s interpretation of some evidence remains in dispute. Prosecutors have noted that the findings are not certain and that no telltale holes from a Taser's muzzle were discovered on Slager's uniform.

While a state investigator said a Taser could not be ruled out as the cause of the tiny burns on the shirt, an expert hired by Slager's lawyers testified that no other known tool has the power to make such unique marks. A Taser can deliver 3,500 degrees of heat through two small areas — the nubs on its end — said Mark Kroll, a biomedical scientist who has worked for the stun gun manufacturer. The electrical burns also make it distinct from other causes.

“It burns in a certain way,” Kroll said. “It’s not like a flamethrower; it’s not like a cigarette lighter. … There’s nothing I can think of … that could have caused that damage besides the Taser electrical weapon being used against Slager’s shirt.”

But whenever witnesses suggested that a Taser was used on Slager, prosecutors pointed out the distance Scott had run by the time the officer started shooting. Assistant Solicitor Chad Simpson, while questioning one defense witness, grabbed Slager’s Taser and said it could not have caused any burns at a few feet away because of a certain mode it was in. The prosecutor walked farther, to the length that was thought to have separated the men when the first shot went off.

“If I had this 15 or 17 feet away with my back turned, would you expect any melting on the shirt?” Simpson said.

No, the witness said.

Focus on forensics

Slager stopped Scott’s car on April 4, 2015, along Remount Road for a broken brake light. Scott, 50, handed over his driver’s license, but as Slager checked the identification, Scott ran.

Giving chase, the officer said he used his Taser to bring Scott down. When he tried to handcuff the man, Slager said Scott yanked the stun gun away. They stood again.

The eyewitness video showed the Taser bouncing on the ground. Scott turned and ran. Five of the eight bullets Slager fired hit Scott.

Slager was arrested three days later, shortly after the bystander’s footage emerged publicly, and his trial has stretched into 14 days of proceedings over four weeks.

While the prosecution’s case focused largely on the video, the defense has turned to its view of forensic evidence, hoping to give the jury a glimpse of what occurred before the eyewitness starting filming.

Gunshot residue, or GSR, is made up of tiny particles spewed from a firearm when it goes off, and it was discovered on Scott’s right hand, not on his left. There are several ways it could have gotten there, said trace evidence examiner Megan Fletcher of the State Law Enforcement Division. GSR can be found on people who are near a gun when it’s fired, who have handled a fired weapon or who have been touched by someone who has used a firearm.

While Slager told investigators that he was concerned about Scott getting his gun during the struggle, he never alleged that Scott actually grabbed his holstered pistol.

Prosecutors also have noted that Slager handcuffed Scott's lifeless wrists after the gunfire, possibly transferring the residue then.

'Can't say ... definitively'

Defense attorneys on Monday sought to refocus most of the jurors’ attention to the Taser. Witnesses earlier in the trial said traces of both men’s DNA was found on it.

As the encounter neared an end, Slager already had fired his two Taser cartridges that shoot probes 25 feet. To shock someone, the Taser could be used only in "drive-stun" mode and pressed directly into a person's body.

Laboratory experts from SLED did tests on what sort of marks a drive stun makes. The agency reported its findings 11 days before the trial.

Fletcher testified that the testing involved shooting a Taser into a polyester shirt, then using a microscope to compare the marks left behind on Slager's shirt. It took at least 480 degrees of heat to melt polyester, she said, and a Taser can exceed that.

"I was not able to rule out a Taser as a possible source of having created those melted fibers," Fletcher said. "I’m not aware of any other source that could create that kind of damage."

Fletcher also tested a clothes iron, which didn't melt the shirt, and a flame, which burned the shirt but caused a color change.

Defense attorney Andy Savage raised, facetiously, questions about more bizarre origins.

"There’s always a possibility that he was laying on the beach with a magnifying glass?" Savage asked. "There’s always that potential of a meteor landing on the earth?"

But Fletcher, when questioned by a prosecutor, acknowledged that the burn marks in the uniform did not look like the small holes she made by using a Taser on the test material. The burns were photographed through a microscope because they cannot be seen with the naked eye. She refused to agree, though, that such a view counted out the Taser from causing the marks.

“I cannot say it definitively did,” Fletcher said. “I can’t say it definitively did not.”

'A psychological shock'

Kroll could offer a more affirmative opinion after his decades of work with electrical weapons, he insisted.

Slager never reported that he felt a shock from the stun gun, but the defense has contended that his ballistic vest could have stopped the electricity.

Poring over SLED's pictures of the burn marks, Kroll said it’s almost certain that a Taser put them there.

“The source of this 3,500 degrees is very small,” he said. “There’s really no alternative source for that heat damage."

To show the jury how a drive stun works, Kroll crammed batteries into a Taser. The first two didn't work. He joked and the jurors laughed about his past mishaps when he mistakenly shocked himself, including one time in a Vermont courtroom.

On the third try, he jabbed the Taser into a canister he held. It gave off a soft clicking sound. When he removed it, a loud noise filled the room as Kroll pointed the Taser in front of the jury box, a bolt of electricity linking two points on the muzzle. The jurors had heard it before, in the bystander's video.

Such a sight and sound can be intimidating, he said. But a prosecutor again added that it would be harmless at a distance, like the one that separated Scott and Slager when the shooting began.

“You can’t electronically shock me,” Kroll said. “It’s a psychological shock.”

Reach Andrew Knapp at 843-937-5414 or twitter.com/offlede.

Andrew Knapp is editor of the quick response team, which covers crime, courts and breaking news. He previously worked as a reporter and copy editor at Florida Today, Newsday and Bangor (Maine) Daily News. He enjoys golf, weather and fatherhood.