NEW YORK — 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 the ease of 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 for counterterrorism agencies. Some experts caution that a limited number of small-scale attacks are likely to continue.

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

Concerns have been intensifying since the rise of the Islamic State group and were heightened this week after two gunmen were shot dead while trying to attack the event in Garland, Texas, that featured cartoon images of the Prophet Muhammad.