Just months after signing on to the Iran nuclear deal, authorities in London reportedly discovered that an Iran-linked group was stockpiling tons of the same explosive used in the Oklahoma City bombing.

In 2015, police in London raided four properties associated with the Lebanese terrorist organization Hezbollah and discovered thousands of disposable ice packs filled ammonium nitrate, a chemical used in rudimentary bombs. During the raids, a man in his 40s was arrested, according to the Telegraph.

Despite the gravity of the discovery, the public and lawmakers were kept in the dark about the matter for years as the U.K. continued to support the nuclear deal. During that time there was also a debate whether to label the entirety of Hezbollah, which has support and funding from Iran, as a terrorist group.

The discovery of the stash was so significant that then-Prime Minister David Cameron and then-Home Secretary Theresa May were briefed on the details personally.

The U.S. branded all of Hezbollah as a terrorist group in the 1990s, but in the U.K. only the armed wing of the group was designated as such until this year.

Sources told the news organization that the discovery of the explosives was not an aberration but was part of an international plot by Iran-backed Hezbollah to lay the groundwork for future attacks. Similar ice-pack explosives have been discovered by authorities in Thailand and in Cyprus.

The U.S. military recently deployed the USS Abraham Lincoln Carrier Strike Group and a bomber task force to the Middle East. The increased military presence comes at the same time the U.S. continues to exert a “maximum pressure” campaign against the regime targeting its financial industry.