Eleven people have been shot dead at the Monte Tchota military barracks on Santiago island, Cape Verde.
Victims reportedly include eight soldiers, a local civilian and two Spanish citizens.
The barracks, off West Africa, house soldiers protecting a communications hub.
The government placed security forces on red alert at the international airport in Praia, the capital, and at the island's ports after the apparent attack, the online service of Expresso das Ilhas said.
A policeman came across the bodies around midday, Cape Verde Television said. It said police later found an abandoned car containing eight Kalashnikovs and ammunition.
Authorities say they believe a disgruntled missing soldier may have been behind the killings.
A government statement said the deaths were not an attempted coup or connected to the drugs trade.
The Cape Verde archipelago, some 370 miles off Senegal, is a former Portuguese colony made up of 10 islands. Around 500,000 people live there.
The country has recently been fighting international drug rings attempting to smuggle cocaine into the country.
Last week police seized 280kg of cocaine from a yacht, and officials have linked two recent armed attacks against public figures to that battle.
A new government took office last Friday following an election in March. It has promised a zero tolerance approach to crime.
Authorities in Cape Verde, which is classified as a developing nation, have received praise and financial aid from international bodies for their commitment to democracy and economic development.