The four marines, who were summoned by the National Investigation Agency, were onboard Italian vessel 'Enrica Lexie' and were present at the spot when their colleagues Massimiliano Latorre and Salvatore Girone allegedly shot dead two fishermen on February 15, 2012.
The four witnesses conveyed that they were not ready to come to India for deposition as witness following summons from the NIA, which is probing the case, official sources said.
The witnesses said that they were ready for appearance either through video conferencing, or a team of NIA should visit Italy for questioning them or the investigators can send them written questions which they would reply.
However, none of the proposals was acceptable to the NIA investigators as Italy was bound to cooperate with India as per an agreement between Rome and New Delhi.
Faced with the four witnesses' refusal to come to India, the Home Ministry has sought opinion of the law ministry on the future course of action, the sources said.
The refusal of the Italian witnesses to come to India is bound to delay the trial against the two Italian marines, who have been at the Italian Embassy here since their arrival following a diplomatic dust-up.
Italy had initially gone back on its promise to return the marines so that they can stand trial in India but later relented.
The two have been slapped with murder charges for gunning down two fishermen Ajesh Binki and Jelestine.
NIA has questioned and recorded the statements of Master of the vessel, Umberto Vittelli, Chief Officer James Mandley Samson, Second Officer Sahil Gupta, Semen Fulbaria Marendra, Kumar Naren and former ordinary seaman Kantamuich Tirumal Rao.
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 waters, 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.
The Italian government had in April reversed its earlier decision not to send back to India the two marines who had gone to Italy to cast votes in elections there.
Italy had reneged on its assurance to the Supreme Court on sending back the two marines but later gave in after the Indian government and the apex court took a firm stand with New Delhi warning that ties with Rome could be downgraded.
External Affairs Minister Salman Khurshid had said in Parliament that the case will not attract death penalty as it was not a rarest of rare case.
In May, Italy had appointed Deputy Foreign Minister Staffan de Mistura as Special Envoy to work out a fair and balanced outcome of the matter.
India has already conveyed its intent to work out a balanced approach.
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