An explosion on the Maldives' presidential speedboat last month was caused by a device placed under the seat usually occupied by the president, who escaped unhurt because he wasn't sitting there, the government said Tuesday.

Home Minister Umar Nazeer told reporters in the capital, Male, that a formal criminal investigation has been launched based on that finding.

Nazeer said President Yameen Abdul Gayoom's usual seat was unoccupied at the time of the Sept. 28 explosion, which injured his wife, an aide and a bodyguard.

"Fortunately the president had shifted and was seated next to the first lady," Nazeer said.

The explosive device "was small, designed not to kill everyone on board. (It was) targeted to kill or incapacitate him (the president)," Nazeer said. He declined to comment on the type of explosive device or how it was detonated.

Nazeer said the details emerged in an investigation conducted by Sri Lankan forensic experts. Investigators from the U.S. FBI, India, Australia and Saudi Arabia have not yet released the results of their probes.

He said up to five people have been arrested in connection with the blast, without giving details.

Gayoom and his wife were traveling from the airport to the capital by boat after making the hajj pilgrimage in Saudi Arabia. The airport is on a separate island in the archipelago state.