New Delhi (CNN) Canadian Prime Minister Justin Trudeau's week-long trip to India has gone from bad to worse.

Already dogged by bad press and speculation that the prime minister had been "snubbed" by the Indian government , on Thursday Canadian officials were again on the back-foot, after it was revealed a Sikh extremist convicted of attempting to murder an Indian politician had been invited to dine with Trudeau at the Canadian High Commissioner's residence in New Delhi.

The official invitation, which was later withdrawn, has sparked outrage in India, where the issue of Sikh separatism remains a highly charged and emotive topic.

"Obviously we take this situation extremely seriously. The individual in question never should have received an invitation. As soon as we found out, we rescinded the invitation immediately," said Trudeau during a gathering of reporters on Thursday

The individual at the center of the controversy is Jaspal Atwal, a Canadian national of Indian heritage, who in 1987 was sentenced to 20 years in a Canadian court for his part in the attempted murder of a visiting Indian state minister.

Complicating matters, multiple Canadian media outlets also carried photographs showing Atwal standing next to Sophie Trudeau, the wife of the Canadian Prime Minister, at a reception held in Mumbai Tuesday.

It is unclear how Atwal, a known militant, managed to obtain a visa to enter into India, or why he was seen traveling with the Trudeau entourage.

Atwal has denied that he is part of the official Canadian government delegation and said he came to India for business, according to Canadian news site the Province.

Speaking to reporters during a weekly press briefing, Raveesh Kumar, the spokesperson for India's Ministry of External Affairs, said he was unable to explain the apparent oversight.

"I can't say immediately how that happened. There are different ways of people coming into India, we are ascertaining details from our mission. We will have to see how this happened" said Kumar.

"The fact that we issued a visa to him, it also means that he's not an Indian national."

A spokesperson for the Canadian High Commission in India declined CNN's request for further comment and referred CNN to the prime minister's public statement.

Atwal was one of four men who ambushed and shot Malkiat Singh Sidhu, a then-member of Punjab's cabinet, who was visiting Canada for a relative's wedding, badly wounding him.

In the sentencing, the judge called the crime "an act of terrorism in order to advance a cause."

According to CBC News , one of several Canadian news organizations to report on the story, at the time of the attack, Atwal was a member of the International Sikh Youth Federation, a banned terrorist group in the US, Canada, the UK, and India.

Perceived 'snub'

Even before Thursday's revelations, Trudeau's week-long trip has generated negative press, due in part to his perceived "snub" by Indian Prime Minister Narendra Modi, who he has yet to meet.

According to an official agenda released by the Indian government, Trudeau, who arrived in India on Saturday, is due to meet with Modi tomorrow in Delhi. But that hasn't stopped speculation in local media, in both India and Canada, linking Trudeau's reception -- or lack thereof -- to his alleged tacit endorsement of Sikh separatists.

Canada is home to about 468,000 Sikhs, comprising 1.4% of its population. A small but influential number of those Sikhs support the idea of Khalistan, or the demand for a separate state for Indian Sikhs.

The separatist issue goes back decades in India, with one of the most important clashes occurring in 1984 when Indian Army soldiers stormed the Golden Temple in Amritsar, Sikhs' holiest site, in an effort to capture separatist leaders. In response, two Sikh bodyguards assassinated India's then leader, Indira Gandhi.

In 1985, the violence spilled over into Canada, when Sikh separatists bombed an Air India plane that had taken off from Toronto airport, killing all 329 people aboard, including numerous Canadians of Indian descent. The only person who was convicted in the bombing was released in 2017 by Canadian courts after serving two decades in jail.

Analysts point to Trudeau's appearance at a Sikh event in Toronto last year, where separatist flags and posters depicting an extremist Sikh leader killed in the 1984 Indian Army operation were displayed, as a particular area of contention.

Speaking to CNN Wednesday, India's former ambassador to Canada Vishnu Prakash said that Sikh separatism was a matter of greater concern inside of India.

"It goes to the core of India's unity and integrity and that is an issue that needs to be sorted out between the two countries (Canada and India)," said Prakash, who described the event Trudeau attended in Toronto last year as "an out and out Khalistani platform."

Trudeau visited the current Chief Minister of Punjab on Wednesday and reaffirmed his support for a united India.