The Canadian inquiry into the 1985 bombing of Air India Flight 182 hit a snag on June 20 when witnesses refused to testify at the last minute, saying they feared for their safety.

The witnesses, whose names were not disclosed, refused to testify even in a closed-door hearing, after officials were unable to guarantee that their identities and statements would not eventually be made public.

"We are disappointed," inquiry spokesman Michael Tansey said, adding that officials did not want to force people to testify against their will if it meant their safety was at risk.

Testimony from another witness scheduled to appear at the hearings in Ottawa also had to be canceled at the last minute because he suffered heart problems on June 19.

Officials did not disclose what the witnesses had been expected to tell the inquiry, or why the people thought the information would put their lives in danger. The number of reluctant witnesses was not disclosed.

The inquiry is investigating Canada's handling of the June 23, 1985, bombing of Air India Flight 182, which killed 329 people over the Atlantic ocean off the coast of Ireland. The mid-air attack remains history's deadliest bombing of a civilian airliner.

A second bomb that went off at almost the same time killed two baggage handlers when a suitcase was being transferred to another Air India flight at Tokyo's Narita airport.

The bombings were believed organized by Sikh militants living in the Vancouver area who were waging a violent campaign for an independent Sikh homeland in India and wanted revenge for India's 1984 storming of the Golden Temple.

There have been allegations that Canada missed chances to prevent the attacks, and then bungled the investigation so the bombers were never caught or convicted.

Police have complained their investigation was hampered by a refusal of members of the Canadian Sikh community to co-operate.

The key prosecution witness in the trial of two men charged with the bombing remains under police protection with the media prohibited by court order from ever disclosing her name. The trial ended with the men's acquittal in 2005.

Tansey said June 20's development was was ironic because one of issues the inquiry was investigating was Canada's ability to protect witnesses in terror-related trials.

"That we have what happened here dramatically illustrates this is an issue," Tansey said.

The inquiry is now on a scheduled break until September when it will look into issues such as Canada's ability to handle long and complex terrorism trials.