Investigations into the bombing of the Air India Flight 182 remain "active and ongoing", the Canadian police have said, terming it the "longest" and one of the "most complex domestic terrorism" probe, ahead of the deadly bombing's 39th-anniversary memorial.

The Montreal-New Delhi Air India 'Kanishka' Flight 182 exploded 45 minutes before it was to have landed at London's Heathrow Airport on June 23, 1985, killing all 329 people on board, most of them Canadians of Indian descent.

The bombing was blamed on Sikh militants in retaliation to 'Operation Bluestar' to flush out militants from the Golden Temple in 1984.

In a statement on Friday, Royal Canadian Mounted Police (RCMP) Assistant Commissioner David Teboul called the bombing the "greatest terror-related loss of life involving and affecting Canadians" in the country's history as he offered "deepest sympathies, understanding and support to the families of the victims".

"The Air India investigation is the longest and certainly one of the most complex domestic terrorism investigations that the RCMP has undertaken in our history," Teboul said. "Our investigative efforts remain active and ongoing," he said.

Asserting that the impacts of the bombing "have not diminished with time" he said that the trauma it caused has impacted generations.

"We must never forget those innocent lives lost to this tragedy and other acts of terrorism," he said.

Noting that this year marks the 39th anniversary memorial of the bombing, he said that the 40th memorial, a significant milestone, is approaching in 2025.

"At the memorials, previous and current employees from the investigation and our greater organisation, past and present, will ensure we remember those lost and those left to deal with the unthinkable," he said.

"Over the years, attending the memorials has been an opportunity to pay our respects to the victims and their families and show our appreciation for all those that did so much to respond to and investigate the tragedy," he said, encouraging people to do the same in private or at the memorials in Vancouver, Toronto, Montreal and Ottawa.

India's consulates in Vancouver and Toronto have planned a memorial service to mark the anniversary of the bombing.

A memorial is scheduled on June 23 at the Air India Memorial at Stanley Park's Ceperley Playground area in Vancouver.

Another memorial service is scheduled at South Lawn, Queen's Park, Toronto.

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