The FBI is investigating men of Somali background who have travelled from the United States to Syria to fight with rebel groups against President Bashar al-Assad.
Kyle Loven, a spokesman for the FBI's Minneapolis office, said on Thursday that in the last few months the bureau had received information indicating that 10 to 15 men from the region's large Somali community had travelled from the Minneapolis-St Paul area to Syria.
Loven said that the FBI believed they would join up self-declared jihadist fighters, although he did not identify the groups, Reuters news agency reported.
The US Justice Department, the FBI and US spy agencies recently assigned coordinators or special teams to monitor such travel, fearing that "radicalised" Americans may return and stage attacks at home.
The FBI inquiry grew out of an investigation opened in 2007 into possible terrorism links by people of Somali background who travelled from the same region to Somalia to fight with the al-Qaeda-linked group al-Shabab, or who raised money for it.
Another US official said most Americans and many other foreigners who had gone to Syria to fight had joined one of the two rival self-declared jihadist groups, the official al-Qaeda affiliate, Jabhat al-Nusra, and the Islamic State of Iraq and the Levant, a former al-Qaeda affiliate which lately had been denounced by al-Qaeda leaders for excessive violence.
The US government recently confirmed that a Florida man, Moner Mohammad Abu-Salha, had become the first known American suicide bomber in Syria.
Security authorities in some European countries, including Britain, Belgium, the Netherlands and Germany, say hundreds of their citizens have done likewise.