Investigators found an unexploded suicide vest in a disco hall in Uganda's capital, suggesting that militants had planned a third bombing during the World Cup final, officials said Tuesday. Four foreign suspects were arrested in connection with the find.

The vest found in a suburb of Kampala on Monday -- with a detonator, wires and ball bearings in a bag like a laptop computer bag -- is consistent with what was seen at the two sites of Sunday's blasts, said the inspector general of police, Kale Kaihura.

On Sunday, hundreds of soccer fans had gathered at the Kyadondo Rugby Club and the Ethiopian Village restaurant in Kampala to watch the televised soccer game; at 10:30 p.m., halfway through Spain's victory over the Netherlands, the bombs exploded.

Investigators tell NPR they believe there were two suicide bombers and a secondary device timed to go off after the suicide blast -- at least three explosions in all. More than 70 people were killed, including Ugandans, Indians, Ethiopians and one American -- 25-year-old aid worker Nate Henn of Delaware.

Al-Shabab, Somalia's most dangerous militant group, claimed responsibility for the attacks. The ultra-conservative Islamists are calling for Uganda to withdraw their African Union peacekeeping forces from Somalia. The group has long threatened to attack outside Somalia's borders, but Sunday's bombings are the first time it has done so.

"I'm startled that they could actually launch an attack outside Somalia," said Robert Rotberg, a Harvard professor who tracks the group and Somalia more generally. "I'm surprised they have done something like this, and it may show just how desperate Shabab has become."

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 are not Ugandan but would not say their nationalities. Kaihura, the inspector general of police, hinted that Somali nationals could be among those arrested. Kaihura also said a Ugandan militant group -- the Allied Democratic Forces -- may have played a role in the attack. Like al-Shabab, the ADF is primarily a Muslim radical group.

A top spokesman for al-Shabab said further attacks would target Uganda and Burundi if the nations continue to supply troops to the African Union-led peacekeeping force in Somalia.

"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."

Ethiopia has also drawn special enmity because it sent troops into Somalia in 2006 to support a weak U.S.-supported government in Somalia. It has since withdrawn its troops. But sources tell NPR there were a preponderance of Ethiopians at the two Kampala World Cup venues that were bombed. Given al-Shabab's dislike of Ethiopians since the invasion, it would have made the two venues enticing targets. It would have allowed al-Shabab to send two messages at once -- one against the Ethiopians and another against Uganda.

Uganda was in the group's cross hairs because it supplies a good number of the peacekeepers who are in Somalia with the AU force battling al-Shabab's efforts to topple Somalia's transitional government. What's more, Uganda is training many of the transitional government's soldiers, a program backed by the U.S. and Europe.

Investigators also say that al-Shabab's strict, Taliban-like dictates may have played a role in its target selection. The group recently banned soccer playing in the areas of Somalia it controls. It also banned Somalis from watching the World Cup, saying it would corrupt good Muslims. Together, those things go a long way toward explaining why the group chose the targets it did over the weekend.

One of the questions the FBI wants answered is who the suicide bombers were. The bureau is helping Uganda with the investigation because a U.S. citizen was killed and other Americans were injured, and the concern is that one of the missing Minneapolis Somalis was one of the suicide bombers.

Two years ago, there was a suicide bombing in Somaliland by al-Shabab. The person who was behind the wheel of the car bomb at that time was a young naturalized American, Shirwa Ahmed -- one of the first young Somali-Americans in Minneapolis successfully recruited by al-Shabab.

Ahmed has the dubious distinction of being America's first suicide bomber. The concern is that al-Shabab is trying to send a message to the Americans, too, by using one of their own.

With reporting by NPR's Dina Temple-Raston and material from The Associated Press [Copyright 2010 National Public Radio]