Eleven days before the 1985 Air India bombing, authorities heard a conversation that suggested Sikh militants might be planning an attack, a former Vancouver police officer told the public inquiry on Tuesday.

Vancouver police probing an unrelated case said they secretly tape-recorded Sikh militants talking on June 12, 1985 about an unspecified event that was to take place in "two weeks."

The conversation was tape-recorded at a meeting in the home of a police informant on June 12, 1985, former Vancouver police officer Don McLean testified on Tuesday.

The first man, believed to be a member of the Sikh militant group Babbar Khalsa, was quoted as saying: "You've not killed an ambassador or consul yet."

The reply from the second man, believed to be a member of the International Sikh Youth Federation, was: "You will see in two weeks we will show the community."

But McLean told the inquiry that he interpreted the conversation at the time to mean the militants might be planning an attack on Indian diplomats working in Canada.

McLean said it was only after the plane was downed on June 23, 1985, that he realized the conversation he heard was likely related to the bombing.

"Once I heard of the explosion. That's the thought I had; that's what they meant," McLean said.

He was requested to attend the RCMP station in Richmond "to provide information to them about my knowledge of various Sikh militants within the community," he said.

But the RCMP's translators who reviewed the tape could not hear the comments or determine who might have uttered those remarks, according to documents introduced at the hearing in Ottawa.

None of the men involved in the conversation were charged in connection with the bombing.

The testimony came as the public inquiry started hearing evidence about a key element of its mandate: whether there was an intelligence failure; and whether Canadian police and security officers did all they could to head off the bombing.

Some 329 people, the vast majority of them Canadian citizens of Indian origin or descent, died when Air India Flight 182 was blown from the sky by a terrorist bomb in June 1985.

The bombing was believed to be the work of Sikh separatists living in Canada who were waging a violent campaign for a homeland in the Punjab region of northern India.

On Monday, the inquiry heard that Vancouver police were told eight months before the bombing that Sikh militants were planning to attack the airline.

Inderjit Singh Reyat is the only person ever convicted, on a reduced charge of manslaughter, for his role in the bomb plot.

Talwinder Singh Parmar, the suspected ringleader and founder of the militant Babar Khalsa sect, slipped out of Canada and was shot dead by Indian police in 1992.

Two more men, Ripudaman Singh Malik and Ajaib Singh Bagri, were acquitted in Vancouver in 2005.

With files from The Canadian Press