Muslim-Americans have repeatedly informed authorities of fellow Muslims they fear might be turning to extremism, law enforcement officials say, contrary to a claim by presumptive Republican presidential nominee Donald Trump this week."They don't report them," Trump said in a CNN interview on Monday, in the wake of the mass shooting at an Orlando nightclub of 49 people by an American Muslim who claimed allegiance to Islamic State. "For some reason, the Muslim community does not report people like this."But FBI director James Comey said, "They do not want people committing violence, either in their community or in the name of their faith, and so some of our most productive relationships are with people who see things and tell us things who happen to be Muslim."It's at the heart of the FBI's effectiveness to have good relationships with these folks," Comey said at a press conference following the Orlando shootings.Andrew Ames, a spokesman for the Federal Bureau of Investigation's Washington field office, told Reuters on Wednesday that the agency has a "robust" relationship with the local Muslim community. FBI agents operating in the area have received reports about suspicious activity and other issues from community members.Michael Downing, deputy chief of the Los Angeles Police Department and head of its Counterterrorism and Special Operations Bureau, said the city's Muslim community has been cooperative in reporting "red flags.""I personally have been called by community members about several things, very significant things," Downing told Reuters. "What we say to communities is that we don't want you to profile humans, we want you to profile behavior."Charles Kurzman, a professor at the University of North Carolina, Chapel Hill, who has conducted several studies on Muslim-Americans and terrorism, disputed Trump's criticism."To claim there is no cooperation is false and defamatory to the Muslim-American community," Kurzman said.