Investigators found an unexploded suicide vest in a disco hall in Uganda’s capital, suggesting that al—Shabab militants planned to carry out more attacks, a government official said on Tuesday. Officials arrested some suspects, but police would not say how many.

The discovery of a suicide vest in a suburb of Kampala on Monday was consistent with what was seen at the two blast sites in Kampala, said the inspector general of police, Kale Kaihura. Officials believe suicide bombers took part in the twin blasts on Sunday night during the World Cup final.

“What we found here is consistent with what we found on both scenes of the crime. And so this is a very significant lead in our investigation,” Mr. Kaihura said.

The death toll from Sunday attack rose to 76, Kaihura said.

Al—Shabab, Somalia’s most dangerous militant group, claimed responsibility for the attacks. The Islamists are calling for Uganda to withdraw their African Union peacekeeping forces from Somalia.

Mr. Kaihura said arrests have been made in connection with Sunday’s attack. He refused to say how many people were arrested and only hinted that Somali nationals could be among the arrests. He said releasing more information could hamper the investigation.

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