A plane carrying 64 alleged mercenaries impounded by Zimbabwe came from South Africa, say air authorities there.
State TV showed soldiers sorting through equipment on the plane
But mystery still surrounds the plane, which while apparently registered with a US company was also reportedly sold to a South African company a week ago.
The intended destination of the men on board - described as burly and white, and militarily equipped - is unknown.
The Zimbabwean cabinet is discussing the issue and has promised more details about the plane and its cargo soon.
The plane has been linked to Equatorial Guinea, which has seen a security crackdown in recent days following reports of a coup attempt.
Equatorial Guinea's information minister told Reuters news agency by telephone that 15 mercenaries had been arrested there, including several South Africans.
The plane registration has been traced to a company in Kansas
He described them as an advance party for the group being held in Zimbabwe.
However, a senior executive of Logo Logistics Ltd which is believed to have chartered the plane, said that the group were heading to eastern DR Congo to guard mining operations there.
"They stopped in Zimbabwe to pick up mining equipment, Zimbabwe being a vastly cheaper place for such," Charles Burrow told Reuters by telephone from London.
The US authorities have denied that there is any connection between the plane and the government, while acknowledging that it may be US-registered.
The Boeing 727-100 was impounded on Sunday after allegedly making a false declaration of its cargo and crew.
A spokesman for the Civil Aviation Authorities in South Africa, Moses Seate, has said the plane took off from the domestic Wonderboom airport, north of the capital Pretoria.
If confirmed, the plane looks likely to have been attempting an illicit journey by making an international flight from a domestic airport.
If the men on board are confirmed as mercenaries, it appears they failed to receive required authorisation from South Africa's National Conventional Arms Control Committee.
That has infuriated the South African opposition Democratic Alliance, which says it is writing to the government to demand more information.
Spokeswoman Raenette Taljaard told the BBC there appeared to be a "gaping hole" in the control of South African airspace.
Speaking earlier to the BBC's Network Africa, Zimbabwe's Home Affairs Minister Kembo Mohadi said the crew and passengers aboard the plane had been detained pending further investigation.
He refused to elaborate on the "various" nationalities of the men or give details about the equipment found on board.
The plane has been moved to a Zimbabwean military base
He said ministers would be trying to ascertain the intended destination of the plane, and would give further details about the plane after the meeting.
Mr Mohadi insisted the detentions were legitimate, because "the fact they did not display their cargo means they have become suspects".
The Zimbabwean authorities say they have put the army on full alert following the incident.
US aviation records show the plane - with a tail number N4610 - registered to an Dodson Aviation in Ottawa, Kansas, but a company official said it was sold to Logo Aviation, a South-Africa-based firm, a week ago.
In Zimbabwe, state television pictures showed army personnel sifting through equipment including sleeping bags, army boots, satellite phones and radios - though