The attempted attack on a provocative cartoon contest in Texas appears to reflect a scenario that has long troubled national security officials: A do-it-yourself terror plot, inspired by the Islamic State extremist group and facilitated through social media. Trying to gauge which individuals in the United States pose such threats — and how vigorously they should be monitored — is a daunting challenge.

Michael McCaul, chairman of the House Homeland Security Committee, said federal authorities are aware of "thousands" of potential extremists in the U.S., only a small portion of them under active surveillance.

Concerns have been intensifying since the rise of the Islamic State group and were heightened this week after two gunmen were killed while trying to attack the cartoon contest that featured images of the Prophet Muhammad.