THIRTEEN months after being gunned down in a row with his neighbour, Irish man John O'Sullivan's accused killer is finally on trial in a rural corner of northern California.

But with the prosecution expected to rest its case on Tuesday, the wife of the deceased has said the legal process has been overshadowed by a deluge of intimidation, provocation and even an effort to put her life and the lives of her family in danger.

O'Sullivan was found slumped over the wheel of his tractor in August last year following an encounter with Ken Zimmerman – he had shotgun wounds in his back and his neighbour was arrested and charged with homicide. The prosecution case at Amador Superior Court in California is now winding down having heard expert evidence from a pathologist, doctors and investigating officers as well as testimonies from witnesses who spoke to the accused while he was in prison. During proceedings, the jury heard extracts of a desperate 911 call and testimony that O'Sullivan had rammed through Zimmerman's locked gate and assaulted him before the fatal shooting.

The relationship between Zimmerman and O'Sullivan was fraught with tension and always threatened to spill over into violence – an intimidating atmosphere that now seems to have infected the trial.

Speaking after giving her own evidence, O'Sullivan's wife, Krista Clem, told the Sunday Tribune of a threat to the lives of her family. Last week she was travelling in her mother's car with her three young children.

"We were getting a lot of noise coming out of the front and we took it to the shop and found out that all of the lug nuts on one of the wheels except one had been taken off; all except the one that you need a key for," she said.

"Now I stay out of the county and I have a special parking space which is being recorded by the courts. The police made out a report. My mom said that she had that tyre on the car for 6,000 miles. She drove up from Texas."

After that they were given an unofficial police escort. They were not informed that this was going to happen but they believe they spotted a series of police cars tailing them.

Clem also claims that a hostile group of Zimmerman supporters have done everything they can to influence proceedings.

"I had all the death threats on the phone. As time went on we showed up [to court] every morning and didn't confront anyone or interact or react to anything on the other side," she said.

"We were doing the very opposite. They were being very provocative and speaking to the jury and the witnesses."

She said a juror was removed after it was discovered she was an acquaintance of Zimmerman's and in spite of her protestations that she could remain impartial. Afterwards she kept talking with the other jurors.

Clem said she is "absolutely concerned" for her safety.

"It's going to be a big relief to get out of here once and for all and not have to deal with this anymore."

Last week she gave evidence to the trial. During cross examination, she said efforts were made to undermine her husband's character.

"I knew I would have to be extremely calm on the stand," she said, adding that every suggestion was thrown her way from her husband's alleged drinking to legal battles she had with neighbours. "Just things that were totally untrue and inflammatory," she said. "John and I always had this creed that the truth is the truth and if you give it enough time it will come out and that was what I was trying to rest on. I know what a good man he was and I know that people who knew him knew that."

The trial continues this week.