The charity Proactiva Open Arms has filed a complaint with 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 Saturday that he also planned to file a separate suit against the Libyan coast guard.
The migrant rescue boat Open Arms docked in Spain on Saturday carrying a woman and the bodies of another woman and a 4-year-old boy, all of whom were found among the remains of a dinghy off the coast of Libya last week.
The boat took four days to arrive in the Spanish port of Palma after finding the migrants adrift about 80 miles (130 kilometers) off Libya's coast, having been abandoned by the Libyan coast guard, 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, said at a news conference.
Open Arms claimed the ship's crew had seen the migrant dinghy but had failed to provide help. Reuters could not find a way to contact the captain of the 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 coast guard 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 their patrol ship, the charity said.
Libya's coast guard disputed the account Tuesday but offered no explanation for how the three migrants came to be stranded on the remains of the dinghy.
The Spanish charity operates in the central Mediterranean, one of the deadliest areas of the sea and favored 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 North Africa.
Open Arms found itself at the center of the European immigrant crisis at the start of the month when it rescued 60 migrants off Libya and took them to Barcelona after being refused docking in Italy and Malta.