On Thursday, according to the United Nations, at least a dozen peacekeepers were killed and dozens more wounded in an attack on the U.N. peacekeeping mission, known by the acronym MONUSCO, in Congo. So far, we know little — but what we do seem to know sketches a daunting picture.

According to local and U.N. sources, around 6 p.m. local time Thursday, unidentified but heavily armed assailants attacked a MONUSCO base near a bridge over the Semuliki River on the Mbau-Kamango road northeast of Beni town. Operated by the Tanzanian battalion of MONUSCO’s offensive combat segment — the Force Intervention Brigade — the base appears to have immediately lost radio contact with other MONUSCO positions in the area.

In what appears to have been an intense battle of around four hours, at least 14 Tanzanian peacekeepers were killed and over 50 others wounded, according to U.N. sources. Several sources have also reported that the attackers also killed at least one and possibly five FARDC (Congolese Armed Forces) soldiers. FARDC sources and local media, however, initially reported that the Congolese army had not been involved. As of Friday, MONUSCO was still evacuating the wounded for emergency medical care.

MONUSCO, the largest U.N. peacekeeping force, took over in 2010 from an earlier U.N. mission. The force’s mission is to support peace and stability in the country, and to protect citizens from clashes with militias fighting for control of the North Kivu region.

While the scale and boldness of this attack certainly comes as a surprise, the “triangle of death” between Beni, Mbau and Kamango has — after a long lull — again seen a string of attacks in recent months. Many of the recent attacks in the Beni region were quickly ascribed to the Allied Democratic Forces (ADF), who were also blamed for most of the attacks in 2014 to 2015. The ADF is an armed group that started in Uganda, in opposition to Ugandan President Yoweri Museveni. Since then, the ADF has morphed into a Tabligh-Islam-inspired armed group operating in Beni’s Ruwenzori area.

ADF’s alleged links to al-Shabab or al-Qaeda have never been independently proved. But the group’s reputation as Islamist militants has for a long time blocked a deeper analysis of its approach, aims and involvement in larger eastern Congolese and cross-border conflict dynamics. The U.N. Group of Experts‘ 2015 report and independent think tanks, such as the Congo Research Group, have released research that suggests today’s ADF is a networked militia that has embedded itself into the Congolese conflict and set up ties with local ethnic militia as well as renegade FARDC units and remnants of the former RCD-K/ML rebellion.

It is quite possible that the ADF is linked not only to the attack on the MONUSCO base but also to much of the increased violence since September, despite FARDC efforts to eradicate the group. The ADF is considered a highly disciplined military outfit. But the most recent mapping of armed groups in the Kivus shows that the ADF is not the only armed actor active in the Beni area. It’s not immediately evident that the ADF alone could have staged such a massive and effective attack on a MONUSCO base.

The next challenge for MONUSCO and the United Nations at-large will be to understand quickly exactly who carried out the killings, which may constitute a war crime — and how a base run by one of the peacekeeping mission’s most robust contingents could be so vulnerable. MONUSCO will focus for the next weeks on shoring up its own security and, inevitably, be less disposed to carry out its mandate to protect Congolese civilians. This might invite follow-up attacks in security voids elsewhere in North Kivu.

As the U.N. and MONUSCO evaluate what happened, they will have to ask whether recent cuts in budgets and troops might have helped provide an easier target for the attackers. And, in light of the unsatisfying U.N. investigation into the deaths of two U.N. workers in Congo earlier this year, the main question will be whether the U.N.’s top leadership has sufficient willingness to launch a thorough investigation.

Christoph Vogel is a PhD student in political geography at the University of Zurich. Find him on Twitter @ethuin.