Tadikira Mafubza the stepson of Malawi’s ex-President Peter Mutharika has been arrested after 30 bodies were found in graves in a forest in October, police said.
The corpses were of boys and men believed to be Ethiopian migrants being trafficked to South Africa.
Police said there was strong evidence linking Tadikira Mafubza to the graves, and a vehicle suspected to have been used to transport the group has been also impounded.
Mr Mafubza has not yet commented.
But the former governing Democratic Progressive Party (DPP) has described his arrest as a political witch-hunt against Mr Mutharika’s family and his supporters.
Police found a mass grave with 25 bodies, and graves with five other bodies, in a forest in the northern district of Mzimba in October after young boys reportedly detected a foul smell.
The bodies were exhumed, but the authorities have not yet disclosed the results of autopsies.
Some reports suggested that the group had suffocated to death while being transported, and they were then buried in the forest.
Malawi is a major trafficking route for migrants from Ethiopia and Somalia, both of which have been hit by instability.
A pathologist, who is part of the team conducting autopsies on the bodies, said partial results had been presented to police and that a full report would be submitted at the end of month.
Police said they had been searching for Mr Mafubza, and he handed himself over at their headquarters in the capital, Lilongwe.
He is the first person to be arrested following the discovery of the graves.
Police did not give more details, and said investigations were continuing.
Malawi is grappling with the problem of organised syndicates trafficking men, women and children to South Africa, the US and Europe.