killed when the twin towers collapsed. and there is tight security in new york and washington this weekend as people gather to mark the anniversary. police and the fbi are on high alert looking for any signs of a terror plot. u.s. officials say they received information of a credible, yet unconfirmed plot to attack those cities. our susan candiotti joins us now from new york. do we know what law enforcement may be looking for? any more details on that potential plot? reporter: yes, fredericka, hello. cnn is learning new details about the timing of this possible conspiracy. at least two of the three men are now believed to have traveled from the mideast region to the u.s. last week. according to a u.s. government official. and a third man believed to be possibly part of this plot was said to be traveling in europe. now, as we ve been telling you, two of those three men are believed to be u.s. citizens. and now we are also learning that there s a further description that the