The poisonous rhetoric of Anwar al-Awlaki, a smooth-talking American-born imam who taught Muslims in San Diego in the years leading up to the attacks of Sept. 11, 2001, has become a constant factor in homegrown terror plots since his death five years ago. “It’s shocking and true that almost every American or European terrorist at some point during their radicalization in some way was influenced by al-Awlaki,” said Julie Shain, director of research for the Counter Extremism Project, a nonprofit aiming to combat radical ideology. Her group’s research found that al-Awlaki has influenced at least 54 extremists in the U.S. and another 34 in Europe.