Immediately after 9/11, experts believed people would only be radicalised by person-to-person contact.
But now there are fears 'digital jihadists' living in the U.S. are being recruited online and are then able to be directed from abroad.
A poll by the Homeland Security Policy Institute at George Washington University revealed a rise in homegrown terrorism cases.
Conducted ahead of the 10-year anniversary of the 9/11 attacks, the survey found there were 21 homegrown terrorism cases between September 2001 and May 2009.
The last two years have seen an alarming rise with 31 cases over the period and more than one every month, according to the Counterterrorism Intelligence: Law Enforcement Perspectives report.
The report said: 'Homegrown and foreign-directed jihadi terrorism and radicalisation are perceived as a real threat by local law enforcement in the United States.'
Police intelligence chiefs from the 56 largest cities in the U.S. were polled for the study, Fox News reported.
They raised their concerns about domestic terrorist attacks and strikes by American citizens orchestrated from overseas. Read More