The nail-packed bomb exploded outside a crowded park in the Maldives capital, Male, on Saturday. It was the first such attack reported in the Maldives, a Sunni Muslim country renowned for its exclusive tourist resorts.
"We condemn without reservation this senseless act of violence," the U.S. Embassy said in a statement.
The U.S. government sent the FBI regional representative to help with the investigation, the statement said without providing details of the aid provided.
"We don't want to reveal too much in this early stage of the investigation," embassy spokeswoman Kathy Fox said.
Government spokesman Mohamed Shareef said the two visiting FBI agents were lending their forensic expertise and would help determine if the attackers had links to international groups.
Maldives police arrested local residents and two foreign citizens in connection with the blast, Shareef said. He declined to identify the foreigners' nationality, but state-controlled media said they were from Bangladesh.
Some of those arrested were cooperating with police, giving information on how the bomb was assembled and who else was involved, Shareef said.
Officials, however, have declined to reveal the motive for the attack, which could damage the country's crucial tourist industry.
"The reason for the attack is now clear," government-controlled Haveeru newspaper quoted assistant police commissioner Abdullah Riyaz as saying. "However, I am not at liberty to make the reasons public at this stage of the investigation."
Riyaz said police were also investigating whether there was a religious or political motive behind the attack.
Police expect to arrest several more suspects, Shareef said. No one has been charged with the bombing yet, but the suspects can be held for seven days before they must face a judge, he said.
A British couple, eight Chinese and two Japanese tourists suffered burns from in the blast. The Chinese and Japanese tourists were discharged from hospital and left the country, local officials said.
The Britons remained hospitalized, but were expected to be released Tuesday, said an official from the British Embassy, who spoke on condition of anonymity because he was not authorized to speak to the media.
"Their injuries are serious, but they are not life-threatening," he said.
Some Western diplomats have expressed concern about the potential for violence in the Sunni Muslim country. Half the population is under 18, reasonably well-educated and with few prospects for good jobs. Some young people have turned to drug use, while others have embraced a conservative strain of Islam that had been virtually unheard of on the islands just a few years ago.
In recent years, there has been tension and occasional outbreaks of violence between opposition activists and government forces controlled by President Maumoon Abdul Gayoom, who has ruled the country for 29 years.
The Maldives, with a population of about 350,000, is by far the wealthiest and most orderly country in South Asia. About 600,000 tourists visit the country each year, accounting for one-third of its economy.