An unexploded suicide bomber's vest has been found hidden in a nightclub in Kampala two days after scores of people were killed in attacks targetting people watching the World Cup final in Uganda's capital
The discovery came as the death toll from Sunday's double bombings rose to 76 and officials in Kampala said their investigations had already led to "a handful" of arrests.
This third site was close to an Ethiopian restaurant that was one of the targets for Sunday's blasts.
"We have established that what was found at the discotheque was in fact a suicide vest, and it could also be used as an IED (improvised explosive device)," said Kale Kayihura, Uganda's police chief.
The would-be bomber may have changed his mind before setting off the charge, and the find was a "significant new lead", he added.
Police said on Tuesday that they had made a number of arrests but officials would not give details or specify the nationality of those detained.
Somalia's militant Islamist group al-Shabaab claimed responsibility for the twin coordinated blasts at two venues packed with crowds watching the World Cup final on Sunday night. The group has threatened attacks outside Somalia and boasts of links to al-Qa'eda.
It was the deadliest terror attack in Africa since the US embassy bombings in Kenya and Tanzania in 1998, which were also blamed on al-Qa'eda.
Kenya has further strengthened security along its porous northern border with Somalia, and Burundi is on high alert following threats from al-Shabaab leaders that it was also a target.
Burundi and Uganda together provided the 6,000 soldiers who make up the African Union's peacekeeping mission in Somalia, AMISOM, widely credited with being the only barrier to al-Shabaab's total takeover of the country's south.
Ugandan opposition leaders, conscious of growing public feeling against the country's deployment to Mogadishu, called for all of its troops to be brought home.
Uganda's President Yoweri Museveni has already ruled that out.
Last week, the regional grouping IGAD agreed to send 2,000 more troops to Somalia, which may have boosted al-Shabaab's motivation to show its capability to take terror to the home soil of nations providing peacekeepers.