On June 23, 1985, an explosion ripped apart Air India Flight 182 en route to New Delhi, killing all 329 people aboard, most of them Canadians of Indian descent.
Authorities believe Sikh extremists fighting for an independent homeland sabotaged the Boeing 747, which crashed into the Atlantic Ocean off the coast of Ireland.
A federal commission of inquiry would conclude that a "cascading series of errors" by police, intelligence officers and air safety regulators allowed the attack to take place.
The complex investigation of the crime was hampered by difficulty raising the wreckage from the ocean floor, agency turf wars and challenges persuading witnesses to come forward.
Talwinder Singh Parmar, a prime suspect, died in 1992. In 2003, Inderjit Singh Reyat pleaded guilty to manslaughter. Two years later a British Columbia judge found Ajaib Singh Bagri and Ripudaman Singh Malik not guilty.
But police have not closed the file.
A "dedicated team" of Mounties continues to probe the attack on the jetliner and a related explosion at Tokyo's Narita airport, said Sgt. Annie Linteau, an RCMP spokeswoman.
"Over the last 30 years, the Air India investigation is the longest and certainly one of the most complex domestic terrorism investigations that the RCMP has undertaken in the history of the Canadian judiciary," Linteau told The Canadian Press.
"We have continuously worked with various international police agencies in Europe, Asia and North America, who have been extremely co-operative."
The Mounties rarely discuss ongoing investigations, and Linteau declined to provide additional details.
Bal Gupta of the Air India Victims Families Association said he had not heard from the RCMP in more than a year.
"I don't know anything about progress," said Gupta, who lost his wife in the bombing.
"In principle, any murder file is not closed unless the culprits are sentenced."
Gupta planned to attend an Air India memorial in Toronto on Tuesday. "For families, we do remember our near and dear ones every of our lives, and on June 23 it becomes even more acute."
His son Susheel planned to attend a ceremony in Ottawa. Other memorials were scheduled in Montreal and Vancouver.
Susheel was also among those present Tuesday in Parliament's Centre Block to mark the national day of remembrance for victims of terrorism. Prime Minister Stephen Harper spoke of doing "everything we can to ensure other families that they will not in the future have to endure the kind of pain and loss that you have."
Susheel regrets that his two young children will not meet their grandmother. "They say time heals wounds. I don't know if it does."
Justice Minister Peter MacKay joined community members and families of Air India victims early Tuesday at a ceremony in Ahakista on Ireland's southwest coast.
The federal inquiry into the bombing, led by former Supreme Court justice John Major, said in 2010 that fundamental changes to intelligence handling, criminal prosecutions and aviation security were needed to prevent another deadly attack in the skies.
The Conservative government has enacted some recommendations and it continues to work on others.
Susheel Gupta applauded the government's investment of $10 million in the Kanishka Project, which has funded research into terrorism.
"I think things are better than they were 30 years ago, and I just hope that things will continue to improve."
— With a file from Mike Blanchfield
Follow @JimBronskill on Twitter