Gardaí face a long and difficult legal battle if they find evidence that Rev George Exoo committed an offence in relation to the death of suicide victim Rosemary Toole Gilhooly.
As detectives prepared to travel to the United States in the next few days as part of their investigation into the controversial right-to-die activist, police in Mr Exoo's home state of West Virginia said they could take no immediate action in the case.
"Without the authorities in Ireland having a reason to question Mr Exoo, we here in the US do not have a reason.
"We cannot detain him.We cannot do anything. We have no probable cause. If he did not commit a crime in the state of West Virginia, our hands are tied," said a spokesman.
A Garda spokesman said arrangements for approaching Mr Exoo in the US were being made through the Federal Bureau of Investigation (FBI).
The FBI would not confirm this. A spokesman said only that a mechanism existed for assisting police authorities in other countries, but would not say whether it was being activated in this case.
Mr Exoo claims in an article on the website for his Compassionate Chaplaincy Foundation that he was once questioned by police in relation to the death of a 79-year-old widow in Florida.
But he was not charged with any offence despite admitting that he secured a bag over her head to aid suffocation.
"Police fingerprinted me and read me Miranda rights but in the end . . . recommended the State Attorney General take no further action," he wrote.
If Irish detectives gain access to Mr Exoo in the US it is likely they will be relying on him to agree to speak to them voluntarily.
He has indicated in media interviews in the US that he does not intend talking to the Irish authorities without clearance from his lawyer.
A file for the Director of Public Prosecutions, who will decide whether extradition proceedings should be initiated, is still being compiled.
Detectives are examining the interviews before completing the file.
They are also awaiting further results from toxicology tests carried out to determine the exact combination of drugs and substances Ms Toole Gilhooly took.
The inquest into her death, due to take place in the Dublin City Coroner's Court in a few months' time, is unlikely to benefit from the evidence of Rev Exoo.
It is believed the Coroner's Act could not be applied in another jurisdiction and, even if it could, the provisions in it governing the compellability of witnesses has been successfully challenged here in the Supreme Court.