The study was released Wednesday by the Triangle Center on Terrorism and Homeland Security, which includes experts from Duke University and the University of North Carolina-Chapel Hill.
Twenty American Muslims were suspects in terror plots last year, compared to 47 in 2009.
The 2009 spike was due mainly to a large number of Somali-Americans who tried to join the al-Shabab militant movement in Somalia.
Last year, five American Muslim terror suspects carried out their plans, including one who tried to set off a bomb in Times Square and others who joined militant movements overseas.
Report on Muslim Americans and terrorism: http://bit.ly/fzi9BH