"There are over 1,200 mosques in the United States, and anywhere from three to eight million Muslims (the figure is heavily disputed, although new studies clearly indicate the number at the lower end of the range). ... Yet for one key reason, the extremists have disproportionate influence. One prominent cleric argued in 1999 that "because they are active they took over…more than 80 percent of the mosques that have been established in the U.S."
...The vast majority of American mosques are funded with Saudi Arabian money, and most of the funders subscribe to the Saudi doctrine of Wahhabism, an 18th-century ideology of extreme purity that supports the spread of Islam through violence. Local imams can be appointed by anyone who chooses to fund and/or found a mosque; hence, the influence of this minority ideology is well entrenched among American clerics."
This is the director of the FBI's response:
"We're not looking for individuals of any particular religion or from any particular country."
FBI Director Robert Mueller - SOURCE