One of the world's leading odontologists said he could not explain the origin of unidentified DNA found on the teeth of two women Robert William Pickton is accused of killing, a B.C. court was told Wednesday.

"I did the tests, I did everything I could possibly do I believe in order to test where this came from and I've not been able to find it," said Dr. David Sweet.

Sweet runs the forensic odontology lab in Vancouver that is world renowned for the methods it has developed to extract DNA from teeth.

Pickton's first-degree murder trial, being heard in B.C. Supreme Court in New Westminster, began in late January. He has been charged in the deaths of Sereena Abotsway, Andrea Joesbury, Mona Wilson, Marnie Frey, Georgina Papin and Brenda Wolfe, among missing women from Vancouver's Downtown Eastside.

In all, Pickton is accused in the deaths of 26 women. He has pleaded not guilty to all the charges. Another trial on the 20 other counts will be held later.

Jurors have previously heard that unidentified DNA was found on one of Abotsway's teeth and on a tooth belonging to Joesbury.

Sweet said he worked extensively with the RCMP lab to try to determine where this DNA could have come from.

Sweet said that the part of the lab where testing is done, referred to him as the "inner sanctum," is only accessed by lab staff who follow strict procedures to prevent contamination.

Every tool, piece of equipment, or material that a bone or tooth could come in contact with is thoroughly decontaminated using a multi-step approach between each sample, Sweet testified.

That multi-step decontamination procedure is also used on the bone or tooth.

Each bone or tooth is then placed in a "freezer mill" filled with liquid nitrogen to reduce the temperature to - 196 C. The tooth is then ground to a powder, which is then analyzed for DNA.

Try to determine origin of DNA on teeth

The Crown and defence raised a number of scenarios with Sweet as to where the unidentified DNA could have come from. They postulated that it could have been on supplies from the manufacturer or that it was due to contamination at the lab.

But Sweet said the decontamination procedures were examined after the fact and yielded no possible hypothesis for what caused mystery DNA to wind up on the teeth.

Meanwhile, an agreement was announced  between the Crown and defence that may reduce the number of witnesses needed.

The agreement means the Crown is no longer obligated to prove that the evidence wasn't tampered with during its collection as the defence agrees that no such tampering occurred.

In instructing the jury on how to handle the admission, Justice James William told them that without such an agreement, the Crown would have had to call every police officer who handled each and every one of the exhibits.

But with the agreement, fewer witnesses would need to be called.

With files from the Canadian Press