As the FBI is reportedly stepping up operations against U.S. activities of the Gülenist Terror Group (FETÖ), an education expert has joined a chorus of criticism of schools linked to Fetullah Gülen, the U.S.-based leader of the terror group, whose 2016 coup attempt in Turkey ended in defeat.

"Why are national news shows unaware that Fethullah Gulen collects $500 million or more from taxpayers for his charter chain, and that his chain is biggest user of H1B visas for Turkish teachers, who take jobs away from US teachers?" wrote Diane Ravitch, a New York University professor and former assistant secretary of education, on Twitter.

Ravitch's remarks are not the first time such charges have been leveled against Gülen.

In early July 2016, U.S law firm Amsterdam & Partners filed a fraud complaint with the Texas Education Agency alleging that Gülen "controls some 150 U.S. charter schools through opaque fronts … Gülen-affiliated Harmony Public Schools has been found to be allegedly engaging [in] widespread abuse of the H1B visa program, misappropriation of public funds, and discrimination against certain students and families."

In 2013, The Washington Post reported that in testimony to a school board in Virginia, a former Gülen school teacher testified that Turkish Gülen teachers "had to turn over 40 percent of their salaries to a secret fund used by the movement."

FETÖ led by Gülen orchestrated the defeated coup in Turkey of July 15, 2016, which left 251 people killed and nearly 2,200 injured.

Ankara also accuses FETÖ of being behind a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police, and judiciary.

FETÖ has a considerable presence abroad, including private schools serving as a revenue stream for the terror group, many of them in U.S. states.

As of 2017, FETÖ operated 136 charter schools in 28 states, and used more than $2.1 billion in U.S. taxpayer dollars since 2010, according to the U.S.' CBS News.