The Air India inquiry took a startling turn on Wednesday after one potential witness reportedly suffered a heart attack and others refused to testify, saying they feared for their safety.

The man who suffered heart problems was taken to hospital, but there was no other information available on his condition.

Former Supreme Court justice John Major hoped to get more information from the mystery witnesses by going behind closed doors, but he said even then they wouldn't talk.

Major said the problem is that he couldn't guarantee the level of confidentiality they're seeking.

"All in all, it leaves the witnesses apprehensive of their safety and that of others if they testify, even at an in-camera hearing, and I can do nothing to pursuade them that is not the case," Major said.

But Major said he simply wasn't in a position to give anyone an iron-clad guarantee that their identities will never be revealed if they testify.

"We do live in a democracy where we're governed by the Charter, where certain rights are defined and where freedom of the press is enshrined," he said. "We live with that and act accordingly."

The hearing has now adjourned for the summer and will resume in September.

There was no official word on the identity of the witnesses who were to speak on Wednesday or the subject matter of their testimony, but it's believed to relate to details of the bomb plot that led to the downing of Air India Flight 182.

An explosion on board brought the plane down over the Atlantic Ocean off the west coast of Ireland in June 1985, killing all 329 people on board. Among the victims were 280 Canadians.

The explosives were allegedly planted by Sikh extremists in luggage that was loaded in Vancouver.

A separate luggage bomb destined for a second Air India flight killed two Japanese baggage handlers at Tokyo's Narita airport.

Talwinder Singh Parmar, the suspected mastermind of the plot, was arrested in November 1985 on weapons, explosives and conspiracy charges, but the charges were dropped for lack of evidence. He died in India in 1992 in what officials said was a shootout with police.

Two other men, Ripudaman Singh Malik and Ajaib Singh Bagri, were acquitted of all charges in March 2005 after the costliest investigation and prosecution in Canadian history.

The uproar over the ruling helped pave the way for the appointment of a public inquiry headed by Major, who is reviewing all aspects of the 1985 attack.

Bomb maker Inderjit Singh Reyat was imprisoned for manslaughter in a 2003 plea bargain.

With files from the Canadian Press