An unexploded suicide vest has been found in Kampala after Sunday's deadly attack on people watching the World Cup final on TV, Ugandan officials say.
They also say that an unspecified number of people have been arrested.
At least 74 people were killed in the twin explosions at a rugby club and a restaurant.
The Somali Islamist group al-Shabab has said it was behind the attack because Uganda is supporting Somalia's government in Mogadishu.
Government spokesman Fred Opolot said the arrests were made after the vest was found in the Makindye area south-east of Kampala.
The vest was found in a black bag similar to those used for carrying lap-tops in a nightclub on Monday afternoon, police say.
It apparently contained ball-bearings, explosives and a detonator.
The nationalities of those arrested has not been released but officials have said a head found near the site of one of the blasts appeared to belong to a Somali national, who may have been a suicide bomber.
The Ugandan police are being helped by American investigators as they try to work out exactly how the explosions were planned and detonated.
BBC East Africa correspondent Will Ross says the Ugandan authorities are under great pressure to show that they can prevent any repeat of Sunday's devastating blasts.
The African Union, which has sent peacekeepers to Somalia, is holding a summit in Kampala later this month and officials say it will go ahead as planned.
In other developments:
- Uganda has started seven days of national mourning, with flags flying at half-mast across the country
- Kenya, which is training Somali government forces, has stepped up security measures along its border with Somalia
- The authorities in Burundi, the other country sending troops to the AU mission in Somalia, are on alert after al-Shabab repeated threats to attack it.
In a statement in Mogadishu on Monday, a spokesman for the al-Qaeda-linked al-Shabab group, Sheikh Ali Mohamud Rage, threatened more attacks in the region.
"Al-Shabab was behind the two bomb blasts in Uganda," Mr Rage said.
Mr Opolot said on Monday that most of those killed - at least 60 people - were Ugandan nationals.
He added that an Irish woman was among those who died at the restaurant, the Reuters news agency reports.
California-based aid group Invisible Children, which helps child soldiers, said one of its workers, Nate Henn, 25, a US national, was among those killed at the rugby club.
Officials said the dead and wounded also included Ethiopian, Eritrean, Indian and Congolese nationals.
Mr Opolot added that the other victims had not yet been identified.
Summit to go ahead
Ugandan Internal Affairs Minister Matia Kasaija told the BBC World Today programme that the severed head of a person from Somalia had been identified at the scene of one blast.
He said Uganda had been aware of the threat, but had been caught off-guard.
About 5,000 African Union troops from Uganda and Burundi are based in Mogadishu, propping up the fragile interim government.
The Amisom force is engaged in frequent firefights with the Islamist insurgents who control much of southern and central Somalia.
Ugandan President Yoweri Museveni condemned the attackers and said his country would not "run away" from its commitments in Mogadishu.
"People who are watching football are not people who should be targeted. If they [attackers] want a fight, they should go and look for soldiers."
But opposition MPs have called for Uganda to reconsider its deployment in Somalia after the blasts.