"Our review of attempted attacks during the past two years suggests that lone offenders currently present the greatest threat," according to the assessment, marked "for official use only" and obtained by The Associated Press.
Recently, terror supporters have encouraged their followers to focus on simple attacks and not complex, elaborate ones like those on Sept. 11, 2001.
"Unlike hardened facilities such as active duty military bases and installations, soft targets such as recruiting stations are more likely to be deemed a feasible target due to their easy, open access to the public," the assessment said.
The agencies also predicted that successful, non-elaborate attacks overseas, such as those where crude car bombs were used, could inspire similar tactics in the U.S.
In audio and video recordings, the suspects in the Seattle case discussed the plot at length, discussing how to time their attack at military recruits, such as by tossing grenades in the cafeteria, the complaint said.