The incident was reported only after the fishermen returned to their village and the Italians had not informed authorities and agencies concerned even after killing the fishermen, they said.
The security detail on the merchant vessel seemed to be trigger-happy as they directly attacked Indian fishermen, the sources said.
As per the norms, any ship coming under attack has to report the incident immediately to the single primary point of contact which is the UK Maritime Trade Operations (UKMTO) in Dubai.
The ships are also supposed to make 'Mayday' distress calls on particular radio frequencies watched by the navies but no such attempt seems to have been made by the vessel.
In case of an attack, merchant vessels are first required to adopt evasive measures and if such a measure does not help, they are supposed to fire warning shorts in air first.
Defence minister AK Antony also termed the incident as unfortunate and serious and said investigations were being carried out by the local police and the Coast Guard in the case.
Noting that strict vigil was maintained by authorities, Antony said, "We want everything as per law... At this stage, saying too much is not correct. But I can tell you law will take its own course."