Canadian authorities have arrested a fourth Indian national over the 2023 killing of a separatist Sikh leader in Vancouver.

Amandeep Singh, 22, was already being held for unrelated gun charges before being charged Saturday with "first-degree murder and conspiracy to commit murder" in the slaying of Hardeep Singh Nijjar.

Three other Indian nationals were arrested this month.

The killing sparked a diplomatic row between Ottawa and New Delhi when Prime Minister Justin Trudeau linked Indian intelligence to the killing.

Nijjar – who immigrated to Canada in 1997 and became a citizen in 2015 – had advocated for a separate Sikh state, known as Khalistan, carved out of India.

He had been wanted by Indian authorities for alleged "terrorism" and conspiracy to commit murder – allegations he denied.

He was shot dead on June 18, 2023, by masked assailants in the parking lot of the Sikh temple he led in suburban Vancouver.

Trudeau announced several months later that Canada had "credible allegations" connecting Indian intelligence to the slaying.

India dismissed the allegations as "absurd" and responded furiously, briefly curbing visas for Canadians and forcing Ottawa to withdraw diplomats.

In November, the U.S. Justice Department charged an Indian citizen living in the Czech Republic with allegedly plotting a similar assassination attempt on American soil.

Prosecutors said in unsealed court documents that an Indian government official was also involved in the planning.

The shock allegations came after U.S. President Joe Biden hosted Indian Prime Minister Narendra Modi for a rare state visit as Washington seeks closer ties with India against China's growing influence.

U.S. intelligence agencies have assessed that the plot on American soil was approved by India's then-spymaster Samant Goel, the Washington Post reported in April.

Canada is home to some 770,000 Sikhs, who make up about 2% of the country's population, with a vocal minority calling for an independent state of Khalistan.