A team of West Midlands Police officers has been to India as part of its investigation into the murder of man in the Punjab area in 2009.
Four British nationals were arrested in July in Smethwick, Wolverhampton and Coventry over the shooting of Rulda Singh outside his home in Patalia.
Mr Singh, in his 60s, was the head of a Sikh organisation.
The visit, led by a Counter Terrorism Unit (CTU) officer, was "very productive", the force said.
Two men in the state of Punjab have already been charged with his murder.
The British men arrested are all aged between 27 and 36.
At the time of their arrests, West Midlands Police said the case was being handled by the CTU because the case had potential links to Sikh extremism.
Detectives had already travelled to India in April.
In a statement, West Midlands Police said: "We can confirm that Supt George Tracey and a team of officers went to India earlier this month.
"The visit was very productive.
"As this is an ongoing investigation, we cannot comment any further."