The witnesses were on board the Italian vessel 'Enrica Lexie' when the marines Massimiliano Latorre and Salvatore Girone had allegedly shot dead two Indian fishermen off the Kerala coast on February 15, 2012.
Sources said three of the witnesses were questioned by the National Investigation Agency, which is probing the case following a Supreme Court order, but a few others have not yet appeared before the investigators despite serving notices.
The Supreme Court had shifted the case to the national capital saying the Kerala Police have no jurisdiction over the case and backed the government's decision to hand over the case to NIA.
Italy had claimed since the incident had taken place in international water, Indian courts have no jurisdiction to conduct the trial.
However, the Supreme Court had ruled that the incident took place at a distance of about 20.5 nautical miles from the coastline of Kerala and, therefore, it occurred not within the territorial waters of the coastline of Kerala state but within the Contiguous Zone.