"There are a number of loopholes for putting people onto the watchlists even if reasonable suspicion cannot be met.
One is clearly defined: The immediate family of suspected terrorists—their spouses, children, parents, or siblings—may be watchlisted without any suspicion that they themselves are engaged in terrorist activity. But another loophole is quite broad—”associates” who have a defined relationship with a suspected terrorist, but whose involvement in terrorist activity is not known. A third loophole is broader still—individuals with “a possible nexus” to terrorism, but for whom there is not enough “derogatory information” to meet the reasonable suspicion standard."
from The Intercept Scahill and Devereaux, "THE SECRET GOVERNMENT RULEBOOK FOR LABELING YOU A TERRORIST"