Baciro Dabo was sleeping next to his wife at around 4 a.m. Friday when the men broke into his residence and opened fire, killing him instantly, according to AP.

Campaigning for the June 28 election was due to begin Saturday, and Dabo had resigned his post as territorial administration minister in mid-May and quit the ruling party to run as an independent candidate in the vote. He was one of the 13 candidates, but was not considered to have any chance of victory.

A special vote pending general elections was organized after the West African country's president, Joao Bernardo "Nino" Vieira, was assassinated March 2, just hours after his long-time rival, the head of the armed forces, was killed by a bomb. Dabo was considered close to Vieira and previously served as his interior minister.

It was unclear what sparked the killing, but a statement issued by Guinea-Bissau's intelligence service said the assassination might have been linked to a failed coup plot it said was being organized by former Defence Minister Helder Proenca.

Rumours were circulating in Bissau that Proenca had also been killed, but officials could not confirm the reports.

Bissau was calm and there were no other reports of violence in the city on Friday.
Ruling-party candidate Malam Bacai Sanha is believed to have the strongest chance of winning the vote.

A former Portuguese colony, Guinea-Bissau has a long history of instability with numerous coups and attempted coups, most of which were led by the army.