WASHINGTON/MALE (Reuters, AFP) - The FBI has said it found no evidence an explosion on board the Maldives president's boat was caused by a bomb, a report said, raising questions about his deputy's arrest over the incident.

President Abdulla Yameen was unharmed in the blast on his speed boat which authorities described as an assassination attempt. The Sept 28 explosion left his wife and two others slightly injured.

Police arrested Vice-President Ahmed Adeeb nearly a month later for treason as he returned to the Indian Ocean archipelago from an official trip abroad.

The arrest followed a series of sackings of government officials including Yameen's defence minister, which have fuelled concerns of political instability and further damaged the Maldives' image as an upmarket tourist destination.

Though the Maldives government initially said the blast could have resulted from mechanical failure, it later cited investigators from the FBI, Australia, Saudi Arabia, India and Sri Lanka to assert it was an attempt on Yameen's life.

Josh Campbell, an FBI spokesman, said in a statement emailed to Reuters on Saturday that the evidence submitted for analysis was determined to be from the boat and not parts of an improvised explosive device (IED).

"Based on the FBI's analysis - which included forensic analysis of the scene, analysis of the items recovered from the scene, and chemical testing - there is no conclusive evidence to attribute the explosion on the boat to an IED," Campbell said.

The Foreign Ministry, on its official Twitter feed, said the FBI forensic report was inconclusive.

Home Minister Umar Naseer said on Sunday that police in the Maldives would have the final say on the investigation. Reports from India and Australia have yet to be received.

"No forensic investigation will ever say conclusively something happened for certain," Naseer told reporters in Male, adding that explosives may not have left traces and evidence could have been tampered with.

A Sri Lankan investigator told Reuters the blast was a "high explosive explosion", while Maldives' local probe commission cited Saudi Arabian investigators as saying there were signs of RDX, "a chemical rated as a high explosive and the main element used in making powerful explosives such as C4."

Foreign Minister Dunya Maumoon on Tuesday denied that dissident politicians were trying to unseat Yameen and said he was in control.

Yameen, 56, has provoked street protests in Maldives with a crackdown on political dissent, including the arrest of the country's first democratically elected president, Mohamed Nasheed, who was sentenced to 13 years in jail this year on terrorism charges that have caused an international outcry.