If those suspicions prove true, it would be the first time that al-Shabab has carried out an attack outside of Somalia.
The explosions ripped through two bars packed with football fans watching the final moments of World Cup in an Ethiopian-themed restaurant and at a gathering in a Kampala rugby club on Sunday.
Ugandan President Yoweri Museveni toured the blast sites on Monday and vowed to bring the attackers to justice: “We shall go for them wherever they are coming from."
Al-Qaeda-inspired al-Shabaab militants in Somalia have threatened to attack Uganda for sending peacekeeping troops to the anarchic country to prop up the Western-backed government.
"At one of the scenes, investigators identified a severed head of a Somali national, which we suspect could have been a suicide bomber," said Felix Kulayigye, an army spokesman.
"We suspect it's al-Shabaab because they've been promising this for long," he said on Monday.
There has been no claim of responsibility for the bombings. But an al-Shabaab commander in Mogadishu praised the attacks. He admitted he did not know whether they were the work of his group, which is fighting to overthrow the Somali government.
"Uganda is a major infidel country supporting the so-called government of Somalia," said Sheikh Yusuf Isse, an al-Shabaab commander in Somalia's capital Mogadishu.
"We know Uganda is against Islam and so we are very happy at what has happened in Kampala. That is the best news we ever heard," he said.
An American aid worker was among those killed and US President Barack Obama, condemning what he called deplorable and cowardly attacks, said Washington was ready to help Uganda in hunting down those responsible.
Ten of the dead were either Ethiopian or Eritrean, a police spokesman said.
Ethiopian troops invaded Somalia in 2006 to oust an Islamist movement from Mogadishu. That sparked the Islamist insurgency which still rages.