A few hours after rebel fighters swept into Goma last week, a mysterious convoy of six trucks rumbled up to the Rwandan border on the edge of the city. They were loaded with "conflict minerals" – including tin and tantalum – from warehouses in Goma.

The potholed streets of this sprawling, refugee-filled city, built on volcanic rock, were largely empty. Most people were huddled inside their shacks or high-walled compounds as the rebels seized the city. But at about 5:30 p.m., just before the frontier closed, the trucks reached the border and the guards allowed them to cross from Congo into Rwanda.

"A convoy of six trucks at the same time is unusual," said Fidel Bafilemba, a conflict-minerals researcher in Goma who received a flood of calls from witnesses when the trucks crossed the border.

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"Rwanda knew the city had fallen to the rebels, yet they allowed those trucks to enter. They should have stopped them."

The M23 rebels have been promising for several days to withdraw from Goma, although the pullout was delayed on Friday when United Nations peacekeepers refused to allow the rebels to take a cache of army munitions and equipment from Goma's airport.

But even if the rebels withdraw, they will leave behind their proxies: a layer of administrators, informers, police and other operatives who will bolster M23's economic power in the city – including their grip on the trade in "blood minerals" that provide revenue to Congo's array of armed groups and can be found in many cellphones and computers around the world.

While the causes of the M23 rebellion are complex, economic factors are among the biggest. By capturing Goma and a large swath of eastern Congo, the Rwandan-backed rebels have assured their influence over the vast mineral wealth of the region – and a wide range of other business activities, from the charcoal and timber trades to gas stations and illicit border revenue.

Global efforts stymied

The rebel victories are a huge setback for the global effort to control the minerals. Activist groups and major electronics companies such as Motorola and Intel had worked for years to establish systems for tracking minerals and certifying them as "conflict-free" – allowing them to be sold legitimately in retail stores in North America and elsewhere.

But those efforts have been heavily damaged by the rebel advances in the past six months. "It has wrecked all the certification and tracing processes," Mr. Bafilemba said.

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The six trucks that crossed the border into Rwanda on Nov. 20 were evidence of two disturbing trends: the rapid growth of mineral smuggling routes from Congo to Rwanda and the strong commercial interests of the M23 rebels who organized or approved the truck convoy.

The rebels have moved fast to exploit their power in Goma. Reports on Friday said the rebels have stolen large sums of money from the Goma branch of Congo's central bank. This week, witnesses saw armed rebels surrounding the bank and removing white bags from it, although there were conflicting reports about whether the bags contained money or food or both.

Many other incidents of looting by the rebels have been reported, including dozens of thefts of cars, trucks and furniture from private homes and government offices. They also helped a businessman recover $10,000 worth of smuggled cassiterite, a conflict mineral, that the police had seized from him, according to a government source who spoke on condition of anonymity.

In another case, according to Mr. Bafilemba, a prominent mineral smuggler was able to break out of prison after M23 captured Goma and the rebels killed a witness who had testified against the smuggler.

Within days of capturing Goma, the rebels began appointing their own loyalists to control the money flow in the city. Last Friday, the government source said, the rebels appointed an official to collect the customs revenue at the main border crossing between Goma and Rwanda, where trucks and cars are queued up every day in a chaotic line. Every night, the rebels send a soldier to collect the daily revenue from the customs official.

It means, in effect, that the flow of minerals is directly supporting the rebels, as it was before the certification efforts. "If minerals can be exported with taxes paid to the rebels, it means they are funding the rebels," the government source said.

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A city in disarray

Three days after capturing the city, the rebel commanders ordered government officials to attend a meeting at the Linda Hotel, where many rebel officers are based. They told the civil servants they could keep their jobs but new "coordinators" would be appointed above them. At the top of the apex, a new provincial governor is expected to be installed by M23, replacing the governor who fled when the rebels captured Goma.

Behind the scenes, the rebel influence is even greater. They can now insist on a share of all legal and illegal trade between Congo and Rwanda, including the mineral smuggling networks. And their recent victories have assured a much freer hand for M23's godfather, General Bosco Ntaganda, who is reputed to have one of the best smuggling networks in the region.

Gen. Ntaganda, known as "The Terminator," is wanted by the International Criminal Court for recruiting and using child soldiers in active combat. Some Congolese say they have seen him in Goma, moving around openly with his wife.

A report last week by United Nations investigators revealed that the mineral smuggling networks are expanding. "The credibility of the mineral tagging system in place in Rwanda is jeopardized by the laundering of Congolese minerals because tags are routinely sold by mining cooperatives," the report said.

"Several traders have contributed to financing M23 rebels using profits resulting from the smuggling of Congolese minerals into Rwanda."

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One of the illicit staging posts for the smuggling networks, according to the UN investigators, is an anonymous-looking building called the Planet Hotel, just a hundred metres from the main border crossing.

The hotel, surrounded by high fences and rolls of razor wire, has few guests these days. Its staff are idle and bored.

But the UN report says the hotel is owned by the husband of one of the leading mineral smugglers, Clemence Rwiyereka Mikamo, who manages a mineral export company in Goma called CLEPAD.

"According to individuals involved in her smuggling operations, in June, 2012, she transferred minerals from her export house premises in Goma to Hotel Planet near the main border post … from where CLEPAD agents hid them in vehicles that crossed into Rwanda during the night," the report said.

The rebels are not alone in their hunger to control these smuggling networks. Congo's national army, too, is deeply involved in illicit business operations. It extracts money from mining operations and other business trade in every region where it is present. The battle between the rebels and military forces in Goma, to some extent, was a battle for economic power.

Congolese generals with official salaries of $80 a month invariably have expensive cars and huge houses, financed by corrupt revenue that flows up from the districts controlled by the officers under their command. Goma was no different. But now the rebels have seized this revenue.

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"War in this country is business," said a UN official in Goma. "It's like the Mafia. With every military operation, people say that the commander must be buying a new house."