US President Barack Obama's request to American Muslims that they must help "root out" extremists in their midst received an angry response inside mosques in a part of California where four men were recently arrested for conspiring to aid Islamist militants.At the West Coast Islamic Society in Anaheim, southern California, where two 24-year-old men arrested in May accused of conspiring to aid Islamic State had worshipped, the message was clear: there is no trust of the Federal Bureau of Investigation or the US government.Imams and workers at mosques also described the arrested Islamic State sympathizers as victims of over-zealous law enforcement, illustrating the difficulty the Obama administration may face in convincing some Muslim leaders to identify and report radicals in their midst to US authorities.They said they found Obama's request insulting, with some questioning why the same message is not sent to Christian churches after mass shootings carried out by non-Muslims."It is unfair to speak about the Muslim community in this way," said Moustafa Kamel, the imam of the West Coast Islamic Center. An immigrant from Egypt 13 years ago, Kamel added, "there is a lot of suspicion of the FBI here."Obama's call for greater self-policing of America's 2.8 million Muslims, made in a rare Oval Office address on Sunday, was followed by a similar appeal from Jeh Johnson, his Homeland Security chief, at a Virginia mosque on Monday. Johnson called it an "ask" of Muslim communities.Obama and Johnson made their appeals after last week's massacre of 14 people in San Bernardino, California, by a U.S.-born Muslim man and his wife, a Pakistani national.The FBI said the two had long been radicalized and conspired to commit terrorism without detection, underlining the challenge of finding so-called homegrown, self-radicalized extremists inspired by Islamic State propaganda.