"India is continuing to violate international law and Italy must keep battling to have the two marines tried by Italian courts," Angela Del Vecchio, a lecturer in international law at Rome's private LUISS university, told Adnkronos.
Under the UN Convention on the Law of the Sea, ratified by Italy, a state may refer a disputed case to an international judge or to third-party arbitration, Del Vecchio noted.
"The problem is that Italy doesn't want to do this. I don't understand why," she said.
India's Supreme Court ruled Friday that the National Investigation Agency (NIA) would continue to probe the case of Italian riflemen Massimiliano Latorre and Salvatore Girone, who are charged with murder over the shooting of two fishermen off the coast of the southern state of Kerala in February 2012.
Italy argued that the case should be investigated by police and not the NIA which probes and prosecute crimes that fall under India's maritime safety act, a legislation which says offences that cause "death to any person" shall be punished with death.
Earlier in April, the NIA charged the two men with murder and violating maritime safety laws.
The Italian government returned the marines to India last month to end an escalating diplomatic standoff, after New Delhi promised Rome that they would not not face capital punishment. Latorre and Girone say they mistook the fishermen as pirates while protecting an Italian oil tanker and Italy claims the incident took place in international waters, meaning India should not have jurisdiction over the case.
There was strong opposition in Italy to the return of the marines for trial by a special court in India and Foreign Minister Giulio Terzi resigned in protest in March.
"This incident has been affected by the Italian electoral campaign and by the fact that Italy is the homeland of (Congress party chief) Sonia Gandhi," said Del Vecchio. She did not elaborate.