The FBI arrested Antonio Martinez, a 21-year-old Muslim convert, Wednesday and charged him with plotting to blow up a military recruitment center. There are two things about this case that make it particularly interesting to counterterrorism officials. The first is that Martinez appears to have been radicalized in the U.S. The second is that he is Latino. Latino converts to radical Islam have been connected to terrorism cases in this country with increasing frequency — and officials are trying to understand why.