Several Tunisian fishermen have effectively become pirates, robbing boats carrying sub-Saharan Africans to the coast of Sicily.

The fishermen reportedly steal the engines of the boats as well as migrants' money and mobile phones, Italian prosecutors in the Sicilian city of Agrigento said on Sunday (July 30).

If passengers of the migrant boats resist, the prosecutors report, the fishermen-turned-pirates threaten them with knives and block their boat until they get what they want.

Several recently reported cases

The prosecutors in Agrigento say they have worked on several such cases over the past few weeks.

Four people from North Africa aged between 43 and 50 have been detained, investigators said. They reportedly are the captain and crew of Tunisian trawler Assyl Salah and were arrested last week by the Agrigento flying police squad, finance police and coast guard from Lampedusa on suspicion of stealing from migrants in the Mediterranean.

A preliminary investigations judge confirmed the arrest orders, ruling that the suspects must be held in pre-trial detention.

The alleged crime of piracy on the high seas breaks the United Nation's 'Convention on the Law of the Sea' and the Italian shipping code. People who violate it face up to 20 years in prison.

Shipwreck survivors blame trawler crew

The four African men have been accused by a number of survivors of a shipwreck off Malta on July 23 in which five people were reported missing, including a child.

Thirty-seven migrants from the Ivory Coast, Gambia, Guinea and Cameroon, as well as the body of a 35-year-old Ivorian woman, were taken to the Italian island of Lampedusa.

The survivors said they had departed from the Tunisian port city of Sfax on July 22 at around 10 pm local time. According to the witness accounts, the migrant boat capsized after being approached by a Tunisian trawler whose crew members tried to steal the engine.

Investigators said another attempted theft was reported at the end of April when a four-year-old girl fell from a migrant ship and drowned -- allegedly because crew members of a Tunisian trawler hijacked the boat on which she was travelling and tried to steal the engine.

The first time migrants spoke about a similar theft was on March 26, people familiar with the matter explained. Then, a seven-meter-long boat carrying 42 people was found adrift in the Mediterranean without its engine. Passengers said crew members of a Tunisian trawler had stolen it.

Half of rescued migrant boats have no engine

According to investigative sources in Agrigento, almost half of the vessels carrying migrants that are rescued at sea are found without their engine.

Tunisian gangs aboard fishing trawlers steal the engines and then sell them to migrant traffickers, they said.

Investigators added that gang members target boats carrying Gambian, Ivorian, Guinean, Senegalese, Sudanese and Burkinabe citizens -- but not fellow Tunisians.

Pirates also try to steal money and mobile phones, the investigators added.

Safeguarding migrants 'fundamental'

Information obtained by investigators on this case has been shared with foreign countries through Interpol, according to people familiar with the matter.

"These arrests show it's fundamental to fight irregular immigration as well as safeguard migrants who end up in the hands of unscrupulous criminals who endanger their lives," said Italy's Interior Minister Matteo Piantedosi.