South Africa’s intelligence services are investigating who was behind a chartered plane that landed in Johannesburg with more than 150 Palestinians from war-ravaged Gaza who did not have proper travel documents and were held onboard on the tarmac for around 12 hours as a result, the country’s president said Friday.

The plane landed Thursday morning at O.R. Tambo International Airport, but passengers were not allowed to disembark until late that night after immigration interviews with the Palestinians found they could not say where or how long they were staying in South Africa, South Africa’s border agency said.

It said the Palestinians also did not have exit stamps or slips that would normally be issued by Israeli authorities to people leaving Gaza.

The actions of South African authorities in initially refusing to allow the passengers off the plane provoked fierce criticism from non-governmental organizations, who said the 153 Palestinians — who included families with children and one woman who is nine months pregnant — were kept in dire conditions on the plane, which was extremely hot and had no food or water.

South African President Cyril Ramaphosa said there was an investigation to uncover how the Palestinians came to South Africa via a stopover in Nairobi, Kenya.

“These are people from Gaza who somehow mysteriously were put on a plane that passed by Nairobi and came here,” Ramaphosa said.