OVERLAND PARK, Kan. — A group monitoring anti-Semitic attacks in the U.S. cautiously noted a sharp decline in such incidents less than two weeks before the fatal shootings over the weekend outside two Jewish sites in suburban Kansas City.

The contrast between the Anti-Defamation League’s 2013 audit and the Sunday attack that killed three people highlights what hate-group trackers say is a broader trend: more overall tolerance disrupted by periodic bursts of violence from a disenfranchised fringe.