The answer is yes: at least 80 percent of them.
Brethren, Mordechai Kedar and David Yerushalmi published a disturbing study today in The Middle East Quarterly that we should read and ponder. The article is entitled Shari'a and Violence in American Mosques. This is an excerpt:
The conclusions to be drawn from this survey are dismal at best, offering empirical support for previous anecdotal studies on the connection between highly Shari'a-adherent mosques and political violence in the name of Islam. The mosques where there were greater indicators of Shari'a adherence were more likely to contain materials that conveyed a positive attitude toward employing violent jihad against the West and non-Muslims. The fact that spiritual sanctioners who help individuals become progressively more radicalized are connected to highly Shari'a-adherent mosques is another cause for deep concern.[26] In almost every instance, the imams at the mosques where violence-positive materials were available recommended that worshipers study texts that promoted violence.
The survey also demonstrates that there are mosques and mosque-going Muslims who are interested in a non-Shari'a-centric Islam where tolerance of the other, at least as evidenced by the absence of jihad-promoting literature, is the norm. Mosques where violence-positive literature was not present exhibited significantly fewer indicators of orthodox, Shari'a-adherent behaviors and were also significantly less likely to promote violent jihad or invite speakers supportive of violent jihad. These non-Shari'a-centric mosques may provide a foundation from which a reformed Islam and its followers can more completely integrate into liberal, Western citizenship.
The results of this survey do not indicate the percentage of American Muslims that actually attend mosques with any regularity, nor does it reveal what relative percentage of American Muslims demonstrate Shari'a-adherent or non-adherent behaviors. Moreover, although this study shows that imams at Shari'a-adherent mosques recommend studying violence-positive materials and utilize their mosques for support of violent jihad, it does not capture the individual attendees' attitudes toward religiously sanctioned violence. However, it is at least reasonable to conclude that worshipers at such mosques are more sympathetic to the message of the literature present at those mosques and to what is being preached there. A follow-up survey of individual mosque attendees would provide insight regarding the relationship, if any, between Shari'a-adherence on the individual level and the individual's attitude toward violent jihad.
Read it all here.