An unexploded suicide vest containing ball bearings was found in a disco hall in Uganda's capital, near the sites of two weekend bomb blasts, suggesting that militants planned another attack, officials said today.
Four "foreign" suspects were arrested in connection with the find.
The discovery of the device in a suburb of Kampala yesterday was consistent with what was seen at the two blast sites, said the inspector general of police, Kale Kaihura.
The vest contained ball bearings, as did the bombs which exploded on Sunday, killing 76 people.
Officials believe suicide bombers took part in the twin blasts - one at an Ethiopian restaurant, the other at a rugby club where World Cup fans were watching the tournament's final at an outdoor screening.
"What we found here is consistent with what we found on both scenes of crime. And so this is a very significant lead in our investigation," Mr Kaihura said.
Four people were arrested in connection with the discovery of the unexploded vest, said Edward Ochom, the director of criminal investigations. He said the four were not Ugandan but would not reveal their nationalities. Mr Kaihura hinted that Somali nationals could be among those arrested.
Al-Shabab, Somalia's most dangerous militant group, claimed responsibility for Sunday's attacks. The Islamists are calling for Uganda to withdraw its African Union peacekeeping forces from Somalia.
The claim by al-Shabab, whose fighters are trained by militant veterans of the conflicts in Afghanistan and Iraq, raises the security stakes in East Africa and has broader implications globally. The group in the past has recruited Somali-Americans to carry out suicide bombings in Mogadishu.
Al-Shabab, an ultra-conservative Islamic group that has drawn comparisons to the Taliban, has long threatened to attack outside Somalia's borders, but the bombings on Sunday are the first time the group has done so.
"We warned Uganda not to deploy troops to Somalia, they ignored us," said Sheik Ali Mohamud Rage, al-Shabab's spokesman. "We warned them to stop massacring our people, and they ignored that. The explosions in Kampala were only a minor message to them. ... We will target them everywhere if Uganda does not withdraw from our land."
Rage said a second country with peacekeeping forces in Mogadishu - Burundi - could soon face attacks. Fighting in Mogadishu between militants and Somali troops or African Union peacekeepers frequently kills civilians.