"Twenty arrests have been made of suspected collaborators, suspected ADF collaborators," police spokesman Fred Enanga told a press conference, referring to the Allied Democratic Forces (ADF) based in the DRC.
"We also have the head teacher and the director of school as part of our inquiries. They need to give us answers to certain questions," he added, without making clear if they had been arrested.
Enanga said the death toll from the grisly raid on the Lhubiriha Secondary School in Mpondwe in a remote area of western Uganda late on Friday was 42, including 37 students.
Another six people were injured and remain in hospital, he added.
Meanwhile families were still desperately hunting for news of their loved ones or facing an agonizing wait for DNA tests on some of the students who were burned beyond recognition in the assault on Lhubiriha Secondary School in Mpondwe, close to the DRC border.
The authorities have said 15 people from the community, including five girls, were still missing.
Joe Walusimbi, the Resident District Commissioner of Kesese, the area where the school was located, said most of the identified victims were buried on Sunday but that there were more funerals on Monday.
"We are almost complete with the burial of the dead already identified and waiting for the DNA tests of those students who were burned beyond recognition," he told AFP.
Joseph Masika, a guardian of one of the missing students, recounted heart-wrenching visits to mortuaries and hospitals.
"We are not sure our children are among those abducted or burned beyond recognition. We are distressed, maybe the government will give us an answer soon and we are praying," the 48-year-old businessman told AFP.
"It's a painful situation no parent would want to go through, but we are keeping hope that they are alive wherever they are."
It was the deadliest attack in Uganda since twin bombings in Kampala in 2010 killed 76 people in a strike claimed by the Somali-based Al-Shabab group.
The Allied Defense Forces (ADF), an armed group historically linked to predominantly Muslim Ugandan rebels opposed to Museveni, has been blamed for thousands of civilian deaths in the DRC since the 1990s.
In a report due to be published this week, U.N. experts said the Islamic State group has provided financial support to the ADF since at least 2019.
The Islamic State (IS) describes the ADF as its regional affiliate, the Islamic State Central Africa Province (ISCAP). I
In March 2021, the United States placed the ADF on its list of "terrorist groups" affiliated with IS.