They were 64 "heavily built men", mostly white. No, they were all black. No, only 40 of them were black.

The plane left South Africa illegally from Wonderboom airport, strayed into Zimbabwe airspace and was ordered down. No, the plane left the country legally, having filed a flight plan to Harare and then on to Burundi. No, the plane was headed for the Democratic Republic of Congo (DRC).

The men on board were suspected of being mercenaries hired to overthrow Zimbabwean President Robert Mugabe. No, they were on their way to overthrow the government of Equatorial Guinea. No, they were going to the eastern DRC to carry out security duties.

These are just some of the stories surrounding the flight of N4610, a Boeing 727-100 cargo plane that has been impounded in Harare.

And 64 - though some reports say there are 67 - of those who were aboard, whether they were white, black or a mixture, and whether they were mercenaries or honest men, are in Harare cells facing intense interrogation.

Not even Frederick Forsyth thought of so many twists and riddles in his Dogs of War, probably the best-known novel about mercenaries.

The book was based on Forsyth's personal adventures - he was involved in an unsuccessful scheme to overthrow the head of state of Equatorial Guinea in 1972.

But even the Dogs of War cannot rival this bizarre tale of confusion.

Some sources say the drama began in November 2003 or December when the company Logo Logistics acquired a fishing concession in Equatorial Guinea and bought or hired fishing trawlers.

"Those guys have never caught a fish in their lives," one source said.

The trawlers were really to be used first to reconnoitre and then to transport mercenaries to oust the government of unpopular President Obiang Nguema Mbasogo in a coup, the sources said. Though part of Equatorial Guinea is on the African mainland, its capital Malabo is on the island of Bioko, and it appears that a seaborne coup was planned, though it is not clear from what staging post it would happen.

Equatorial Guinea and its immediate neighbouring island state of Sao Tome and Principe have become ripe for coups since oil was recently discovered in their waters. That has made them big prizes for greedy politicians and those who help them to acquire power. Sao Tome experienced a coup in 2003, which was reversed by African Union intervention.

On Tuesday the Mbasogo government announced that it had arrested 15 "mercenaries" in Malabo, including white South Africans, black South Africans of Angolan origin and a few people from Kazakhstan, some Armenians and a German.

"It was connected with that plane in Zimbabwe. They were the advance party of that group," Information Minister Agustin Nse Nfumu said. He said the 15 had been in the country since December.

The arrests in Malabo corroborate the account of South African security sources that the real destination of the plane seized in Harare was Equatorial Guinea, though other destinations have been claimed.

South African civil aviation sources say Harare was on the aircraft's official flight plan - en route to Burundi.

On Tuesday, a company named in connection with the flight disputed all the speculation, saying the "mercenaries" were in fact security people "going to eastern DRC".

They were stopping in Zimbabwe to pick up mining equipment, "Zimbabwe being a vastly cheaper place for such".

Charles Burrow, a senior executive of Logo Logistics which had chartered the Boeing 727 freighter, said via telephone from London that most of the people on board were South African and had military experience, but were on contract to four mining companies in the DRC. He declined to name the companies.

How then did the crew file a flight plan to Burundi?

Perhaps the most murky leg of the journey was the detour to Zimbabwe, and there are several different explanations offered of how the plane came to land in Harare.

One was that the Boeing 727-100 strayed into Zimbabwe airspace by accident - either through a navigational error or a technical fault - and another that it flew there deliberately.

Several Zimbabwean aviation sources initially said the plane was forced to land after entering Zimbabwe airspace illegally. They said the Airforce of Zimbabwe (AFZ) were alerted once the plane showed up on radar screens. The AFZ has been on alert for years because of Mugabe's fears that his enemies might mount an attack to topple him.

Sources behind this theory say that, upon being questioned, the crew indicated they had wanted to refuel urgently so they could proceed with their journey. The plane was then allowed to land.

A problem arose when the crew were told that the plane was going to be searched, according to this theory. They panicked and tried to taxi off the runaway but the attempt to escape did not succeed. If it had succeeded it probably could have caused a disaster as there was a plane arriving from Johannesburg on the same route.

However, Burrow, the Logo Logistics executive, said the aircraft was bound for the DRC to do mine security work and had stopped in Harare to pick up mining security equipment.

A senior Zimbabwean aviation official, involved in the investigations, said the suspected mercenaries had indeed informed their interrogators in Harare that they had been hired by a South African firm for a noble mission to do both mining and demining security work in the DRC.

The official said Zimbabwean police, army and intelligence officials were unconvinced by this explanation as many of the suspects had shown little knowledge of demining work during interrogation. It was possible that some of the equipment seized was used for mining work.

It is known, however, that it was at Wonderboom Airport that the crew picked up its passengers, and interesting cargo.

Wonderboom Airport manager Peet van Rensburg confirmed that the plane landed there early on Sunday morning and departed later that afternoon with 64 men on board. It is not clear whether this number included the three-man crew, but the flight plan filed indicated 67 people were on board.

Of these, at least 20 are believed to be South Africans, with the majority coming from Pretoria and Johannesburg.

According to Van Rensburg, who was phoned by the duty airport manager, the plane landed without prior notice, and took on passengers and cargo.

But Craig Partridge of Air Traffic and Navigational Services (ATNS) said four flight plans for the plane, registration number N4610, had been filed with ATNS's briefing office in Johannesburg.

According to the documents, he said, the plane left Lanseria Airport at 6.55am on Sunday. There were four crew members on board and they were headed for Wonderboom Airport in Pretoria, where they touched down at 6.59am.

Van Rensburg said that when he arrived at the airport, he found the plane parked on the maintenance runway.

He was told that scores of bags containing various "military-like" equipment had been loaded onto the plane by the crew after the ground crew's help was refused.

Army-style duffel bags contained night vision equipment, waterbottles, apparent gun cases, cylindrical metal tubes, camouflage uniforms and boots, and two-way radios.

His inquiries also revealed that the plane had a limited amount of fuel on board as the Wonderboom Airport runway, which is 1,83km in length, is too short to allow a plane with full fuel load and cargo to take off.

Civil Aviation Authority (CAA) spokesperson Moses Seate said the CAA was conducting a probe into the circumstances of the plane's "illegal" departure from South African airspace.

But a flight plan filed with ATNS indicates that the plane flew to Polokwane, an international airport, from where it would proceed to Harare.

The plane arrived at Polokwane at 4.35pm. It parked on the main international apron in front of the international departures building which houses customs and immigration, Partridge said.

It then took off at 6.24pm, still with 67 people on board, according to the paperwork, and headed for Harare.

"The air traffic controller on duty saw the men getting on board and he described all of them as 'non-white'," said Partridge.

South African air control handed the plane over to air traffic control in Harare when it crossed into Zimbabwean airspace at 6.40pm.

Partridge said a fourth flight plan on file shows that the aircraft planned to leave Harare and fly on to Bujumbura, Burundi. However, it was seized by Zimbabwean authorities.

A Zimbabwe official confirmed that the aircraft had, in fact, entered Zimbabwean airspace legally on Sunday night after filing an earlier flight plan. A problem arose when the crew made a false declaration of its cargo and passengers.

A crew member allegedly attempted to bribe an airport security official with wads of US dollars to avoid a search of the aircraft. That only raised the anxiety of other security officials who witnessed the bribery attempt, and a search was then mounted.

South African security sources offer another explanation - they also say the aircraft flew deliberately to Harare, but with the purpose of picking up Simon Mann, the head of Logo Logistics and leader of the team heading for Equatorial Guinea, as well as some other members.

"But how they thought they could get away with that, I don't know," one source said.

"The age of coups is past now, they must realise that. You could probably take Equatorial Guinea with five people but the African Union is not going to let a coup stand," he added.

He said Nigerian troops had already been sent to Malabo to protect Mbasogo's government.

In 1999, the Organisation of African Unity passed a historic resolution outlawing coups and resolved to banish from the organisation any government that came to power by coup.

The men on Flight N4610 have been detained, their plane impounded, and very little further information about the investigation is being released.

It is understood the suspected mercenaries have been split up and are being held at different places around Harare - Chikurubi Maximum Prison, Harare Central Prison, Harare Central Police cells, Rhodesville Police Station cells and army barracks around Harare.

They are being interviewed by different groups of interrogators, say sources.

There is some concern among legal rights activists that the men may be tortured to extract information.

It has not yet been confirmed whether any of the 64 arrested men on board are, in fact, South Africans.

South African High Commissioner to Zimbabwe Jerry Ndou said his office had written to Zimbabwe's ministry of foreign affairs to ask for information about the detainees. "Our interest is to confirm whether or not any South Africans have been arrested," he said.

"But part of the problem is that the cabinet is now in session and so we have to wait for a reply. Anyway, we are hoping they will come back to us as soon as possible," he added.

Commenting on the delays in obtaining information, Ndou said: "I have written to the ministry, and so there will be consultations with the minister, the deputy minister, the administrator, the chief director, all those people. It can be that at this stage we are just waiting on the minister Stan Mudenge, who is in cabinet."

Ndou said he needed to know the identities of the detainees so that he could inform their families at home, and also offer them consular services.

According to Ndou, the impounded plane has "made big news in the state media", with reports stating that Zimbabwe was under siege.

Equatorial Guinea is equally paranoid, believing that the "mercenaries" were meant for a coup there.

Its information minister, Nfumu, said from Malabo that the suspected mercenaries had arrived in the former Spanish colony, which borders Gabon and Cameroon, in December and were picked up late on Monday evening. He said some of them had been "presented to the diplomatic corps".

The clampdown comes amid growing tensions within Mbasogo's family, whose members hold most top positions in the country, and speculation among exiled opposition politicians that a coup was in the offing.

But this could all be wrong: Logo Logistics, the company that leased the plane on Harare's tarmac, said it was carrying 64 mining contractors to the DRC. It said that what had been described as "military" items on board were, in fact, equipment such as boots, and pipe-bending and wire-cutting tools.

"We can make it clear that we have no current or intended business in Zimbabwe and certainly no illegal intentions against its government and people," the company said in a statement sent to Sapa.

It said the aircraft was recently purchased and still registered in the United States. "There is no other link with the US," the company said.

So what's the solution to all the mysteries? Easy. Frederick Forsyth just needs to write Dogs of War II.