Police on Tuesday arrested 20 people, including the headmaster of the school in which 42 people, mostly children, were killed in a massacre on Friday, they said.
The detainees are suspected of collaborating with extremists, the police said.
Uganda’s police blamed rebels of the Congolese militia Allied Democratic Forces (ADF), which is said to have links to the Central African offshoot of the Islamist terrorist militia Islamic State, for the bloodshed.
On Friday, the suspected Islamists attacked and set fire to a school and its dormitories in the town of Mpdonwe near the border with the Democratic Republic of Congo (DRC).
Almost all of the fatalities were children. According to Ugandan authorities, eight other pupils suffered serious burns and gunshot wounds.
Some of the students were killed with machetes, the military said.
In addition, the attackers abducted 15 girls and boys. Their fate is still unknown.
The military said it had pursued the attackers across the border into the DRC.
There, the fighters fled to the Virunga National Park, Africa’s oldest national park, where several rebel groups have been operating for years.
Although brutal violence is a recurring occurrence in the region, the massacre was met with horror abroad.
United Nations Secretary General António Guterres denounced the attack and demanded the immediate release of the abducted children.
Those responsible for this horrific act must be brought to justice, he said.