A series of bomb threats against multiple Indian airlines has rocked the country this week, throwing authorities there and across five other nations into a scramble to investigate threats amid long delays and flight diversions.

India’s flagship carrier, Air India, has been the target of more than a dozen warnings since Monday, along with threats targeting local private carriers.

So far, the threats have turned out to be hoaxes. However, they come amid heightened tensions between India and Canada over the long-term presence of Sikh separatists in the country.

It’s a fatal combination: Sikh separatists, India, and Canada have a deadly history linked to one of the worst aviation disasters ever recorded.

Here’s what you need to know about the bomb threats and why they matter as India and Canada feud:

What bomb threats have been received?

At least 19 bomb threats have been issued on social media sites like X against aircraft in the country in the space of three days, including Air India flights and three private carriers.

Bomb threats are common, but several targeting a country in a few days is unusual.

On Monday, officials disclosed that three international flights were diverted or delayed following threats of bombing, including an Air India flight from New Delhi to Chicago, which was forced to land in a remote airport in Canada. On Wednesday, Canadian officials deployed an Air Force plane to airlift the 200-plus passengers to Chicago.

Another series of threats followed on Tuesday, including one targeting an Air India Express flight from Madurai in southern India to Singapore. The Southeast Asian nation’s air force scrambled two F-15SG fighter jets to escort the aircraft to a sparsely populated bay at Changi Airport, said authorities.

Several threats were also recorded on Wednesday, targeting private carrier IndiGo’s flights from Mumbai to Singapore, Mumbai to New Delhi, and Riyadh to Mumbai. Two SpiceJet flights and another Akasa flight were also affected.

Air India said on Monday that it had received several threats in recent days. Indian authorities also say they are investigating the incidents. At least one minor has been arrested in connection with some of the threats, officials say.

On Wednesday, India’s aviation minister, Ram Mohan Naidu Kinjarapu, condemned the threats in a statement on X. “I am deeply concerned over the recent disruptive acts targeting Indian airlines, affecting the domestic and international operations. Such mischievous and unlawful actions are a matter of grave concern, and I strongly condemn any attempts to compromise the safety, security and operational integrity of our aviation sector.”

Why are these threats historically significant?

On June 23, 1985, an Air India flight going from Canada to India via London exploded around the Irish coast, killing all 329 people on board. There were 307 passengers – mostly Canadian citizens of Indian origin – and 22 crew members. It was the worst aviation disaster in the world before the 9/11 attacks on the United States. It remains Canada’s worst air disaster to date.

On the same day, another bomb went off in a Tokyo airport, killing two Japanese baggage handlers. The bomb was believed to have targeted another Air India flight to Bangkok before exploding prematurely.

Investigators blamed Sikh separatists who were believed to be seeking revenge for the Indian Army’s deadly storming of the Golden Temple in Punjab state a year earlier. At the time, the movement for a separate Sikh nation formed from Indian Punjab — called Khalistan — had gained momentum in India. Since then, the Khalistan movement has largely died in India, but has strong support among sections of the Sikh diaspora in Canada, the US, the UK and Australia.

Although several people were accused and charged over the 1985 bombings, only one was convicted: Inderjit Singh Reyat, a British Canadian electrician, served sentences in Canada and the UK between 1991 and 2016. He is now a free man.

Talwinder Singh Parmar, a Canadian Sikh separatist leader, was tried alongside Reyat but later released because the prosecution couldn’t prove its case.

Canadian police in 2000 also arrested wealthy Vancouver businessman Ripudaman Singh Malik, and Ajaib Singh Bagri, a mill worker from British Columbia, on charges including mass murder and conspiracy. Both were also released.

Families of the victims have long accused Canadian authorities of not doing enough to arrest the perpetrators. In 2010, a four-year inquiry by Canadian investigators found that authorities had mishandled the investigations and that a series of errors had allowed the attacks to happen.

Have there been more recent threats?

In late 2023, Gurpatwant Singh Pannun, a Sikh separatist leader based in the US, warned people in a video posted on social media to stay away from Air India flights beginning November 19, 2023. The warning came after reports emerged of a foiled assassination plot to kill him on US soil.

Pannun, an immigration lawyer, is a leader of the US-based Sikhs for Justice group (SFJ), which is banned in India. He is known for his vocal and violent separatist advocacy for Khalistan and for anti-India videos on social media, where he sometimes encourages people to deface the Indian flag or scribble anti-India graffiti. He has been charged with terrorism and conspiracy in India. In October 2023, he threatened to carry out a ‘Hamas-like’ attack on India, in the wake of Hamas’ deadly attacks on Israel.

An Indian secret service agent directed a plot to assassinate Pannun, according to an indictment released by the US Department of Justice last November. The operative was also linked to a separate high-profile murder of a Sikh separatist leader in Canada – also in 2023.

The US operation was foiled after Nikhil Gupta, a New Delhi businessman hired by the Indian agent, unknowingly partnered with a US secret service informant to carry out the murder, leading to his arrest, according to US authorities. Gupta’s lawyers are expected to meet prosecutors on Thursday to seek a plea deal.

What’s the current India-Canada crisis about?

Tensions ignited by the bomb threats come amid a deepening spat between India and Canada that has seen both sides expel or withdraw dozens of diplomats over the past year.

In September 2023 Canadian President Justin Trudeau, speaking in parliament, first accused India of the killing of Sikh separatist leader Hardeep Singh Nijjar, who was fatally shot outside a temple in Surrey, a Vancouver suburb, on June 18.

India’s government dismissed the allegations as “absurd” and asked Canada instead to crack down on the anti-India groups operating from its territory. For years, India has accused Canada of harbouring Sikh separatists, although Canada denies it.

Sikh groups have carried out unofficial referendums on Khalistan in Canada. Nijjar, 45, was wanted in India for an alleged attack on a Hindu priest in the country.

Four Indian nationals, whom Canadian authorities say are linked to a notorious Indian crime boss, have been arrested for his murder.

Why have tensions escalated?

On Monday, the spat escalated after Canadian officials expelled India’s High Commissioner, Sanjay Kumar Verma, and five other Indian diplomats, accusing them of involvement in the Nijjar assassination and other “acts of violence” against Sikh separatists.

Trudeau said the Canadian police had uncovered “clear and compelling” evidence of Indian government involvement in Nijjar’s killing and targeting of others.

“This includes clandestine information-gathering techniques, coercive behaviour targeting South Asian Canadians, and involvement in over a dozen threatening and violent acts, including murder,” Trudeau told reporters at a news conference.

On Wednesday, the Canadian prime minister doubled down on the accusations during an inquiry on foreign interference in the country, saying India had made a “horrific mistake” violating Canada’s sovereignty.

India on Monday similarly expelled six Canadian diplomats, including the acting high commissioner of Canada in Delhi, Stewart Ross Wheeler, according to a statement from India’s Ministry of External Affairs of India.

The country’s foreign office said that it had “received a diplomatic communication from Canada suggesting that the Indian High Commissioner and other diplomats are persons of interest” in the ongoing investigation into Nijjar’s assassination.

“This latest step follows interactions that have again witnessed assertions without any facts. This leaves little doubt that on the pretext of an investigation, there is a deliberate strategy of smearing India for political gains,” its statement read.

“India now reserves the right to take further steps in response to these latest efforts of the Canadian Government to concoct allegations against Indian diplomats.”

Vina Nadjibulla, research director at the Asia Pacific Foundation of Canada, told Al Jazeera that Monday’s expulsions marked “a serious escalation in the diplomatic tensions” which have now gone on for over a year.

“We’re seeing a diplomatic rupture,” said Nadjibulla. “Canada is continuing to call on India to cooperate with the investigation, but India has refused to do that.”

“What will be decisive in this is the views of our key allies and partners,” the analyst added, referring to a possible response by the US and other Group of 7 nations, of which Canada is a member.