Nearly half of American Muslims say Muslim leaders in the United States have not done enough to challenge Islamic extremists, a survey found. The Pew Research Center survey showed 48 percent of U.S. Muslims don't believe American Muslim leaders have spoken out enough against Islamic extremists, compared with 34 percent who said the Muslim leaders have done so. By contrast, 68 percent said American Muslims as a whole are cooperating as much as they should with law enforcement in fighting terrorism. The survey found U.S. Muslims overwhelmingly rejected suicide bombings and other violence against civilians, with 81 percent saying violence is never justified. One percent said violence is often justified to defend Islam from its enemies and 7 percent said suicide bombings are sometimes justified for that reason. According to the survey, 60 percent said they are very concerned or somewhat concerned about a possible increase in Islamic extremism in the United States while 35 percent they are not too concerned or not at all concerned. The survey found 21 percent of U.S. Muslims said there is a great deal of support for extremism in the American Muslim community and 6 percent believed there was a fair amount. The survey also revealed rising anti-al-Qaida sentiments, with 70 percent of U.S. Muslims holding a very unfavorable view of the terrorist group, compared with 58 percent in 2007. Five percent had a very, or somewhat favorable, view. The survey was based on telephone interviews of about 1,075 people and had a sampling error of plus or minus 5 percentage points.