According to the Fars News Agency, the head of Iran's Justice Department said that the activists were arrested "for transferring intelligence to foreigners, and it is likely that more activists will be arrested".
Tehran prosecutor Abbas Jafari Dolatabadi claimed the NGO was set up "about a decade ago" as a cover to collect "classified information in defense and missile fields".
The death of an imprisoned Canadian-Iranian environmental activist in Tehran has led to growing criticism over Iran's treatment of detainees, including concerns about torture in confinement, during a recent crackdown on antiregime dissent.
In an interview with RFE/RL's Radio Farda, the lawyer for the Seyed-Emami family, Payam Derakhshan, said it had seen CCTV footage showing Seyed-Emami in his jail cell.
Farideh Oladghobad, Iranian educator and reformist politician who is now a member of the Parliament of Iran representing Tehran, Rey, Shemiranat and Eslamshahr electoral district, refused to charge environmentalists with espionage.
Tahbaz, who is a member of the Strategic Committee for Protecting Iranian Cheetah, is a dual Iranian and American citizen. "We were suspicious of the route they chose", Firuzabadi told ILNA news agency (via Middle East Eye).
"You have constantly urged dual nationals to go back to Iran and serve their homeland".
"Because there is not a good angle. the act of suicide is not clear", he told ILNA.More news: Matt Graham wins silver in Pyeongchang
The news was broken out after one of the accused spies of the dossier, Kavous Seyyed Emami, committed suicide while in detention in Evin prison of Tehran.
"Apparently it has been thought that Mr Seyed Emami committed suicide there but there is not enough evidence".
Members of the family have warned against the conflicting accounts being spread about Emami's death, but have not spoken directly to the media.
Emami's son Ramin, a well-known singer, said on Instagram that the family had launched a legal complaint over the death.
Kaveh Madani was also arrested and briefly held over the weekend.
A well-known water conservation activist trained in the United States, Madani was plucked in September from his job at Imperial College London to join the government, signaling President Hassan Rouhani's efforts to encourage the return of Iranians from overseas.
The Center for Human Rights in Iran (CHRI), a non-profit group based in NY, said at least nine other staff members and executives of the Persian Wildlife Heritage Foundation were arrested the same day as Seyed-Emami, citing information from a relative of one of those arrested.
The vice-president at the time Mohammad Ali Abtahi, stated she died from "a brain haemorrhage caused by a beating".