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.
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.
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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.
The two have been slapped with murder charges for gunning down two fishermen Ajesh Binki and Jelestine.