The satellite photos were damning. In the dense forest, under the shadow of a volcano, a well-trodden trail can be clearly seen from the sky, leading directly from a Rwandan military base across the border to a headquarters of Congo's M23 rebel militia.
The 15-kilometre trail, up to four metres wide, was used to transport military supplies and recruits from the Rwandan military to the rebels who dramatically captured the key city of Goma in eastern Congo this week, United Nations investigators say.
The satellite images are among dozens of pieces of evidence gathered by UN investigators revealing how Rwanda violated an arms embargo to give direct help to the M23 rebel militia in the Democratic Republic of Congo, allowing the rebels to seize a huge swath of the mineral-rich region and severely weaken Congo's national government, which the rebels have vowed to topple.
"There are no economic or migratory activities which would otherwise explain the existence of such trails," a team of UN experts noted dryly in a report on their investigation.
"If they are not frequently used, the dense forest would cover these trails in a little over a month," it noted.
Rwanda's crucial support for the M23 rebels is the latest chapter in a long and bloody history between Rwanda and Congo. Since the late 1990s, its tough and battle-hardened military has played a key role in supporting a series of Congolese rebel groups, allowing it to gain strong influence and economic power in the eastern regions of one of Africa's biggest and most war-torn countries.
Its aid to the rebels, ranging from troop reinforcements and weapon supplies to financial and logistical support, is pivotal in explaining the stunning advances of the M23 rebel group in recent months.
The rebels, with an estimated 3,000 fighters in their ranks, are far from a typical ragtag militia. Wearing crisp, new camouflage uniforms, and armed with sophisticated equipment such as night-vision gear and 120-millimetre mortars, the rebels swept into Goma this week, overwhelming the city's defences, humiliating the Congolese army and pushing aside the UN peacekeeping force in the strategic city.
The rebels are now surging southward, toward Bukavu, another strategic city and the capital of South Kivu province. On Thursday, they were advancing steadily on a road to the south, forcing Congo's army into retreat. By capturing Bukavu and Goma, the rebels would be in a strong position to dominate most of eastern Congo.
Rwanda has denied the UN allegations. But there is a growing international consensus that the conclusions of the UN investigators are correct. In a key breakthrough on the issue, the British government announced on Thursday that it accepted the accuracy of the UN report and it threatened to cut off future aid to Rwanda as a result.
"We judge the overall body of evidence of Rwandan involvement with M23 in the DRC to be credible and compelling," said a statement by British Foreign Secretary William Hague.
"We will be studying the implications of this report in full, but these allegations will necessarily be a key factor in future aid decisions to the government of Rwanda."
The 204-page report of the UN investigators was presented on Wednesday to the UN Security Council, which is wrestling with how to respond to the rebel capture of Goma. The United States, a close ally of the Rwandan government, has vetoed any attempt to name Rwanda specifically as the key supporter of the rebels, but that stand might now be changing, according to some reports.
The UN investigators also found evidence that the rebels had received military help from Uganda, which – like Rwanda – has a history of interfering in eastern Congo. But the biggest assistance to the rebels has come from Rwanda, the report said. It said the rebels are unofficially under the command of Rwanda's Defence Minister, General James Kabarebe, along with an indicted war-crimes fugitive, General Bosco Ntaganda – known as "The Terminator."
The evidence cited in the UN report is detailed and exhaustive. It includes radio intercepts of communications between Rwandan and rebel commanders, scores of interviews with former M23 rebel fighters (including 26 Rwandans), photos of grenades used in Goma that are routinely used in the Rwandan army but not used by Congo's army, and captured Rwandan identity documents from Rwandans who were conducting intelligence operations with the rebels in Congo.
"Rwandan officials have provided military support to M23 through permanent troop reinforcements and clandestine support through special forces units," the report said.
The investigators also found that M23 rebels have been recruited in Rwandan villages, while former members of other militias have joined the rebellion by travelling through Rwandan territory, and funds for the rebels have been collected by members of Rwanda's ruling party.
"Officers of the Rwandan armed forces have also furnished the rebels with weapons, facilitated the evacuation of casualties to Rwanda and shared communication equipment with M23," the report said.