Gülenist Terror Group (FETÖ) leader Fetullah Gülen is a threat not only for Turkey but also poses a massive national security problem for the U.S., according to Robert Amsterdam, an international lawyer Ankara hired regarding the investigation into the group.

Speaking at the official launch of the Turkish translation of his book on FETÖ, "Empire of Deceit: An Investigation of the Gülen Charter School Network," Amsterdam called FETÖ the largest violator of H-1B visas in U.S. history.

"Many of the teachers employed in Gülen charter schools were not qualified; some of them were even criminals. And Gülen schools also do not provide a proper education as their graduates almost never get proper degrees at universities," he said.

Gülen schools previously came under fire for irregularities in applications for H-1B visas, which allow U.S. employers to employ foreign workers in specialty occupations, and employing inexperienced Turkish teachers was cheaper than employing U.S. citizens. Amsterdam also said that it is not possible for a "Turkish imam without a sophisticated Western education" to penetrate the U.S. education system without the assistance of "certain agencies" in the U.S.

Ankara accuses Gülen of masterminding the coup attempt on July 15, 2016, which was foiled by pro-democracy protestors as well as military and police forces loyal to the government. Some 249 civilians were killed and 2,000 more wounded in the incident nationwide.

Gülen has lived in self-imposed exile on a 400-acre property in the foothills of the Pocono Mountains in Pennsylvania, U.S., since 1999.

Explaining that his firm had not received funding from the Turkish government for the research for the book, Amsterdam said they hired their own researchers and experts to dig out facts about Gülen schools, adding that it was very difficult to prove the existence of his network due to its secrecy.

Amsterdam's book, which was published on Sept. 13, 2017, describes how the schools are connected to a web of management, real estate and supply companies in a "closed loop system." It also documents over $243 million worth of fraud by the group through its charter schools in the U.S.