Why are some Sikhs calling for a separate homeland in India?

Diplomatic tensions over calls for a separate homeland for Sikhs in India have escalated, after Canada said it was looking at "credible allegations potentially linking" the Indian state to the murder of a separatist leader in British Columbia.

India has denied the allegations, describing them as "absurd".

For anyone new to this long-running controversy, here's an at-a-glance look at the history and current context: