Interesting. From the Article:
A draft document obtained by The Associated Press concludes that citizenship is an "unlikely indicator" of terrorism threats to the United States and that few people from the countries Trump listed in his travel ban have carried out attacks or been involved in terrorism-related activities in the U.S. since Syria's civil war started in 2011.
Weren't these the same countries Obama restricted?
If so, that would mean fewer people to serve as a source group.
How many terrorist activities have been disclosed in the US? (relatively few).
After all, if some guy named Muhammed goes running through the local mall shooting his AK, wearing a bomb vest, and yelling "Alahu Akbar!" we'd be told the "Motive is uncertain at this time".
So even one person is significant, considering the relatively low number of incidents.