Twitter has come under criticism from some analysts who say the social media company has failed to swiftly remove accounts that recruit potential terrorists and incite violence, raising concerns that the United States has not done enough to combat the Islamic State’s rapid expansion of its propaganda operations online.

Mark Wallace, CEO of the Counter Extremism Project, said on Wednesday that the Islamic State (also known as ISIS or ISIL) terrorist group has effectively used social media sites such as Twitter to propagandize and radicalize individuals, including Americans. His nonprofit project recently chronicled 66 U.S. citizens who are accused of joining or attempting to join the Islamic State, plotting attacks in the United States, providing financial support to extremist groups, or disseminating radical propaganda.