Iran claimed Monday it has arrested 17 Iranian nationals allegedly recruited by the Central Intelligence Agency (CIA) to spy on the country’s nuclear and military sites, caught after they were exposed concealing messages in stones left in parks and on mountains to be retrieved by foreign intelligence operatives.

Some of those arrested have already been tried and sentenced to death. The allegations of spying come as Iran and the U.S. continue to spar over who was responsible for the recent string of oil tanker attacks in the Gulf region.

Previous arrests of those accused of espionage against Iran have included accusations that spies used an eagle, a dolphin, a vulture, a lizard, and Mossad-inspired shark attacks against assorted Islamic state targets.

An Iranian intelligence official told a press conference in Tehran the arrests occurred over the past months. He did not say how many received the death sentence but claimed a CIA ring in the Islamic republic “has been crippled” and a secret method of hiding messages in fake stones revealed. He said:

CIA has used intelligence concealment containers to transfer safe communication tools and super advanced spywares for its network in Iran. The most of these containers had been prepared in shape of stones and those mentioned tools were embedded inside and spies have been assigned to retrieve them from different urban districts like parks, mountains and so on by using a complex procedure. Using containers, CIA has also sent fake identification documents for its spies on order to be used for special operations.

The official did not give his name but was identified by the official IRNA news outlet as the director of the counterespionage department of Iran’s Intelligence Ministry. Such a procedure is highly unusual in Iran; officials usually identify themselves at press conferences.

The official said some of those arrested worked in nuclear and military sites. He claimed none had succeeded in their sabotage missions.

Iran Releases Photos of CIA Officers Handling Spieshttps://t.co/xeuAJ5J6wE pic.twitter.com/etLnfiPfOF — Fars News Agency (@EnglishFars) July 22, 2019

⚡️⚡️⚡️ Coming soon on @PressTV: The Mole Hunt ⚡️⚡️⚡️#Iran has dealt a heavy to a @CIA spy network in the region and beyond. The upcoming documentary will shed more light on Iran’s counter-intelligence operations. pic.twitter.com/mlQ3e8aqft — Press TV (@PressTV) July 19, 2019

This is not the first time Iran has accused foreign spies of using secret methods of concealment for their nefarious activities.

Last year foreign agents were accused of using lizards and assorted small reptiles to “attract atomic waves” and spy on Iran’s nuclear program, the former chief-of-staff of Iran’s armed forces claimed.

Previous allegations against spies have also accused them of using an eagle, a dolphin, a vulture, and Mossad-inspired shark attacks on various Islamic foes – with varying degrees of success.