Forty one migrants are thought to have died in a shipwreck last week in the central Mediterranean, Italian authorities and United Nations agencies said on Wednesday, citing survivors who have been taken to the Italian island of Lampedusa, Reuters reported.

Local public prosecutor Salvatore Vella and three U.N. agencies confirmed media reports that four people who survived the shipwreck had told rescuers they were on a boat carrying 45 people, including three children.

The survivors – a 13-year-old boy, a woman and two men – arrived in Lampedusa on Wednesday, almost six days after the sinking of their boat, the International Organization for Migration (IOM), Unicef and UNHCR, said in a joint statement.

The boat had set off on Aug. 3 from Tunisia’s Sfax, a hot spot in the migration crisis, but capsized and sank during the night after being hit by a big wave, the survivors were quoted by multiple sources, including Ansa news agency, as saying.

The Italian Red Cross and the Sea-Watch charity rescue said the four had survived by hanging on to life jackets or other inflatable rubber devices and then finding another empty boat at sea, on which they spent several days adrift.

According to Reuters the migrants arrived in Lampedusa exhausted and in a state of shock and are due to be questioned by police, prosecutor Vella said. They are presumed to have had no food or drinking water until their rescue on Tuesday.

Vella, who has opened an investigation, said they were picked up after a surveillance plane of the EU border agency Frontex spotted them about 54 nautical miles (100 km) off Zuwarah in Libya.

The central Mediterranean is one of the world’s most dangerous migration routes. More than 22,000 people have died or gone missing in its waters since 2014, according to the IOM.

The U.N. agencies said migrants who set off from Tunisia in recent days faced “prohibitive weather and sea conditions”, making their journeys on unseaworthy iron boats “disproportionally dangerous”.

The agencies reiterated a call for governments to dedicate more resources to Mediterranean search and rescue missions – an expensive and politically sensitive endeavour for which there is little appetite in EU capitals.

On Sunday, the Italian coast guard reported two other shipwrecks, with 57 survivors, two dead and more than 30 missing, and media reports said they also involved at least one vessel that had departed from Sfax on Aug. 3.

A source with knowledge of the matter said the latest migrant sinking was probably a separate incident.

The coast guard did not respond to a request for comment, read the report.

Separately, Tunisian authorities said on Monday they had recovered 11 bodies from a shipwreck near Sfax on Sunday, with 44 migrants still missing from that incident.

Italy, a major route into Europe for hundreds of thousands of asylum seekers and other migrants, has seen some 93,750 arrivals by sea so far this year, interior ministry data shows, up from about 44,950 in the same period last year.