A judge declared a mistrial in the case of murder suspect Tony T. Callihan after an expert witness refused to testify.

Callihan, 40, is accused of strangling his girlfriend, 48-year-old Michelle Koenen, on Dec. 7, 2012. He was found a week later in Clearlake, California, where police located him sleeping in Koenen’s car.

His attorney sought a mistrial after his expert witness said he was no longer able to testify, for undisclosed reasons. The trial has been underway since Dec. 1, and there was no way to move forward without the witness, Reid argued.

Prosecuting attorney Dale Nagy objected, asking the judge to force the witness, Dr. Carl Wigren, to say why he suddenly can’t testify.

“What’s the conflict?” Nagy said. “I don’t see anything here.”

Wigren was supposed to rebut the prosecution’s expert witness as to Koenen’s cause of death, Reid said. “They had homicide by ligature; our doctor was suicide,” he said.

Spokane County Superior Court Judge James Triplett granted the mistrial because of the witness issue and other problems in the case.

The prosecution failed to disclose 18 letters written to and from Callihan in prison until this week.

The letters involved four people, two of whom were on the witness list. The defense only received the letters two days ago, Reid said.

There were also allegations that one of the jurors may have discussed the case with others and talked about it on social media, Triplett said.

The cumulative issues are “just too much to ignore,” Triplett said.

A new trial is set for February.