Officials say a second generation of Somali immigrants is becoming increasingly radicalised and could pose a growing threat to security.
The warnings come amid the revelation that 20 young Somali American men who returned to their war-torn homeland have been radicalised by a group linked to al-Qaeda.
The FBI is urgently examining links between the youths, who are all American citizens, and al-Shabaab, an Islamist group fighting in the country's long-running conflict.
Investigators are concentrating on two mosques, the Abubakar As-Saddique Islamic Centre in Minneapolis and the Dawah Institute in the neighbouring city of St Paul, where parents and guardians of the departed youths said their sons attended classes.
But the probe has now spread to Boston, San Diego, Seattle, Columbus, Ohio and Portland, Maine.
US law enforcement agencies are concerned young militants could return to the US to plot terror attacks, following a similar path to the British Pakistanis behind the London bombings in July 7, 2005 who made multiple visits to radical mosques in Pakistan. Read more...