The DNA match of Thomas Mair to the weapons used to kill Mrs Cox in a West Yorkshire street was “one in a billion”, an expert said.
The sawn-off shotgun used by Mair, 53, was splattered in the MP’s blood, it was said.
The dagger used to stab Mrs Cox 15 times, which damaged her internal organs, was also smeared in her blood, the Old Bailey heard.
Mair is accused of murdering the MP in front of horrified onlookers outside a library in Birstall in Mrs Cox’s constituency of Batley and Spen on June 16.
During the attack, 77-year-old Bernard Kenny tried to jump on Mair’s back to “take him down”, the court heard.
But Mair plunged the dagger in the pensioner’s stomach, who then staggered across the road to a sandwich shop, it is claimed.
He then resumed his attack on Mrs Cox and stabbed her again, jurors heard.
Hilary Parkinson, a DNA expert, said Mrs Cox’s tights had holes in the knees suggesting she had tried to crawl away from her assailant.
There were also blood-stained fingermarks on a car parked outside the library which matched the MP’s DNA, she told the court.
Photos of the gun were shown to the jury.
Tom Little, prosecuting, said: “We have heard expert evidence about this, that the barrel and stock had been cut down.
“I think you found numerous blood stains on the rifle?”
She replied: “I did, yes.”
The DNA profile from the gun had a “one-in-a-billion” match to Mair, the court heard.
DNA from the blood found on the gun also had a one-in-a-billion match to Mrs Cox, Ms Parkinson confirmed.
Ms Parkinson said: “The blood was airborne, but because there were only a few spots around that location, we couldn’t determine exactly how they were deposited.
“For example, if somebody is shot, then blood can be projected back onto the weapon.”
However if the weapon was near someone exhaling blood through their airwaves, or had blood in their mouth, “that could also project spots on to the weapon”, she told the court.
But she added: “What I can say is that the source would be close to the weapon.”
A bloody handprint was also found on the back of Mair’s brown jacket recovered nearby in Union Street, which matched Mrs Cox’s DNA, jurors were told.
Ms Parkinson said: “It would suggest she had made contact with the jacket at a time when her hand was wet with blood.”
None of Mr Kenny’s DNA was found on the knife, the court heard.
Prosecutor Tom Little asked the witness: “Does that suggest, in your view, that the knife was used to stab Helen Joanne Cox again after it had been used to stab Bernard Carter Kenny?”
She replied: “It does, yes. There was no evidence of blood. I would have expected some.
“It would suggest that the knife had been used on Helen Joanne Cox again.”
Ms Parkinson examined the outside of the library on the day Mrs Cox was killed, along with the body.
Her tights were “extensively laddered” and there were holes in the knees.
Mr Little said: “Did you form the view that Helen Joanne Cox had been crawling or struggling along the ground, resulting in that damage to her tights?”
She confirmed that she did, adding that “she had not been up very long while she was bleeding”.
Photos were shown to the court of bloody fingermarks on the nearby car which had matched Mrs Cox’s DNA.
Mr Little asked: “Was there also heavy blood staining which had pooled along the bottom edge of the rear of the vehicle?”
She said: “There was, yes.”
It suggested Mrs Cox had touched the car after sustaining “at least one injury, such that her hands were heavily stained in blood”, the court was told.
Mair was armed with British-made hollow-point rifle bullets more commonly used to kill vermin in a “humane” way, the trial heard.
A plastic bag found in a holdall when the alleged killer was arrested was found to contain 25 live .22 calibre rounds, jurors were told. Twelve of the rounds were lead hollow-point cartridges made by British maker Eley, firearms expert Andre Horne told the trial. The other 13 were made by a German firm.
Mair shouted out “Britain first” and “keep Britain independent” as he attacked the pro-EU MP, and later told police he was a “political activist”, the Old Bailey heard.
In the days leading up to the attack he accessed extreme far-right material, including some about the Nazis, and white supremacists, on library computers, it is claimed.
Mair, of Birstall, West Yorkshire, denies murder, grievous bodily harm with intent, possession of a firearm with intent to commit an indictable offence and possession of an offensive weapon - namely a dagger.
The trial continues.