The charity Proactiva Open Arms yesterday filed a complaint - including accusations of involuntary manslaughter - with the Spanish police against a cargo ship for failing to help migrants adrift on a destroyed dinghy in the Mediterranean.
The captain of the charity's rescue boat said yesterday that he also planned to file a separate suit against the Libyan coastguard.
The migrant rescue boat Open Arms docked in Spain yesterday carrying the bodies of a woman and a four-year-old boy as well as one woman who was found alive floating on the remains of a dinghy off the coast of Libya last week.
The boat took four days to arrive in the Mallorcan port of Palma after finding the migrants adrift about 80 miles (130km) off Libya's coast after being abandoned by the Libyan coastguard, the charity said.
"We have filed a complaint against the captain of the [merchant ship] Triades for failing to help and for involuntary manslaughter and we'll also do it against the captain of the Libyan patrol," Oscar Camps, the Open Arms' captain and founder of the NGO, told journalists at a news conference.
Open Arms claimed the ship's crew had seen the migrant dingy but had failed to provide help.
Reuters could not find a way to contact the captain of Triades, which flies a Panamanian flag. The ship is currently moored in the Libyan port of Misrata, where officials could not be reached for comment.
The Libyan coastguard also left the three migrants to float amid the shattered remains of the raft after the two women and the boy had refused to board its patrol ship, the charity said.
Libya's coastguard disputed the account last Tuesday, but offered no explanation for how the three migrants came to be stranded on the remains of the dinghy.
Rome and the European Union have trained and financed the Libyan coastguard to halt the flow of migration across the Mediterranean.
The Spanish charity operates in the central Mediterranean, one of the deadliest areas of the sea and favoured by people smugglers operating out of Libya.
Charity boats have been locked out of Italian ports, the closest European landing point, since Italy's new government vowed to crack down on illegal immigration from northern Africa.
Open Arms found itself at the centre of the European immigrant crisis at the start of the month when it rescued 60 migrants off the coast of Libya and brought them to Barcelona after being refused permission to dock in Italy and Malta.
The rescuers had refused to dock in Italy, saying they didn't trust how Italian authorities would handle an investigation into the wreckage of the dinghy.
The country's populist interior minister, Matteo Salvini, has repeatedly denied entry to aid ships rescuing migrants in the Mediterranean, including Open Arms, and referred to the group's claims and account of its rescue operation as "lies and insults".