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A four-year inquiry into the 1985 bombing of an Air India flight is due to be published. The inquiry was ordered after the acquittal of two suspects and allegations of a bungled investigation by the Canadian police and intelligence services.

Air India flight 182 from Canada to India blew up off the Irish coast, killing all 329 people on board - most of them Canadian citizens visiting relatives in India - on 23 June 1985.

Around the same time, a second bomb exploded prematurely in Japan, killing two baggage handlers.

The bombings - widely believed to have been carried out by Canadian-based Sikhs in retaliation for India's deadly 1984 storming of the Golden Temple, the holiest shrine in the Sikh religion - remain Canada's deadliest terror attack.

But the trouble began even before 23 June.

On 4 June, members of the Canadian secret services followed two men to some woods on Vancouver Island.

They heard a loud explosion, but did not regard the incident as important.

Five months later the two men, Talwinder Singh Parmar and Inderjit Singh Reyat, were arrested on various weapons, explosives and conspiracy charges in connection with the Air India disaster.

However, the case against Parmar turned out to be flimsy, and charges were dropped. Reyat was fined on a minor explosives charge.

Parmar, regarded by the Royal Canadian Mounted Police (RCMP) as the mastermind of the Air India bombing, was killed by police in India in 1992.

Reyat ultimately pleaded guilty to manslaughter in connection with the Air India bombing, and was sentenced to only five years in prison.

Widespread expectations that he would testify against Ripudaman Singh Malik and Ajaib Singh Bagri - the two men acquitted in 2005 - proved unfounded.

After charges against Parmar were dropped in 1985, the next embarrassment for the investigation came with the news that the Canadian secret services (CSIS) had destroyed tapes of telephone calls made by Sikhs suspected of involvement in the Air India case.

The judge in the 2005 trial described this as "unacceptable negligence" but it was regarded as far more serious than that by members of parliament calling for an inquiry.

In 2000 a former secret services officer told the Toronto Globe and Mail newspaper he had destroyed the 150 hours of tapes rather than hand them over to the Mounties, because he feared the identity of informants would be revealed.

And according to RCMP documents, the CSIS ordered the destruction of the wiretaps to conceal the fact that one of its agents had infiltrated a circle of Sikh extremists planning the attack.

He was ordered to pull out three days before Air India Flight 182 blew up.

The next blow for investigators came in November 1998, when a Sikh publisher, Tara Singh Hayer, was killed at his home.

Mr Hayer was to have been a prosecution witness in the Air India trial, and had already been paralysed in an earlier attempt on his life.

Ajaib Singh Bagri, a mill worker from Kamloops, BC, and Ripudaman Singh Malik, a wealthy Vancouver businessman, were accused of planting the bombs on board the plane.

But the prosecution case turned on the reliability of key prosecution witnesses, who claimed the accused had privately confessed to involvement in the bombing.

One was a woman who had been employed by Mr Malik. The pair had been close, but Mr Malik later fired the woman (given anonymity by the court) after reportedly calling her a "slut".

The judge concluded that she had "not been truthful" with the court, saying he found it hard to believe her when she said that she still loved and respected Mr Malik, despite testifying against him.

Another witness, an FBI informant from New York referred to as "John", was controversially paid $460,000 to testify against Mr Bagri.

He said he needed the money to protect himself and his family against possible retaliation.

Another witness who came forward with evidence implicating Mr Malik at a very late stage, after the trial had started, admitted that he was in serious financial difficulties, but denied that the hope of collecting a reward had inspired him to give evidence.

After the acquittals, the judge said the prosecution had failed to prove its case in what had been one of the longest and most expensive trials in Canadian history.

The inquiry was ordered in June 2006 by the then Prime Minister Stephen Harper amid outrage from the public and victims' families over the events.

Carried out by Air India Commissioner John Major, the inquiry has also examined aviation security, the financing of terrorism and Canada's ability to handle major terror-related trials in future.