The Allied Democratic Forces (ADF), accused of the attack last Friday in the western Ugandan town of Mpondwe, has been blamed for thousands of civilian deaths in neighboring Democratic Republic of Congo.
In a report due to be published this week, U.N. experts on the DRC shed light on the hazy financial connections between the two groups.
The Islamic State (IS) has "provided financial support to ADF since at least 2019, through a complex financial scheme involving individuals in several countries on the continent, emanating from Somalia and going through South Africa, Kenya and Uganda," it said.
The evidence comes from documents and testimony, it said.
Historically linked to predominantly Muslim Ugandan rebels opposed to President Yoweri Museveni, the ADF established itself in eastern DRC in the mid-1990s.
It became one of scores of outlawed forces in the troubled region, but in 2019 pivoted towards radicalism, pledging allegiance to the IS.
The IS describes the ADF as its regional affiliate, the Islamic State Central Africa Province (ISCAP). In March 2021, the United States placed the ADF on its list of "terrorist groups" affiliated with IS.
In November 2021, Ugandan troops crossed into eastern DRC, joining Congolese forces in a crackdown on the ADF after a string of bombings in the Ugandan capital Kampala.
Military officials on both sides proclaimed Operation Shujaa to be a success, but attacks by the group have continued.
The new U.N. report says the ADF "sent combatants and/or collaborators on scouting missions" to try to expand beyond North Kivu and Ituri provinces.
The group "sought to recruit and conduct attacks in Kinshasa as well as Tshoppo, Haut-Uele and South-Kivu provinces," it adds.
The attack in Uganda occurred less than two kilometers from the border with the DRC.
Officials said 37 students at the Lhubiriha Secondary School were killed - 17 in the torched men's dormitory, and 20 female students who ran, but were hacked to death.
Four non-students were also killed.
Major General Dick Olum, who has been leading the Uganda side of the operation against the rebels in the DRC, told AFP that intelligence suggested the ADF had been in the area at least two days before the attack.
They were apparently on a scouting mission to plot the raid on the school, he said.
"We have called for more firepower, planes to help in the rescue operation of those abducted, and locating the rebel hideouts for military action," he said.
In June 1998, 80 students were burnt to death in their dormitories in an ADF raid on Kichwamba Technical Institute, also near Uganda's border with the DRC