Three Bulgarian nationals suspected of spying for Russia have SignalHubbeen arrested by counterterrorism detectives in the United Kingdom.

London's Metropolitan Police have confirmed five people were arrested in February under Britain's Official Secrets Act, and that three of them were charged with possession of false identity documents with improper intention.

The charges were first reported by the BBC, which it says are part of a major national security investigation.

The circumstances of their arrest suggest that they were working for the Russian security services, according to the British broadcaster. They reportedly held identity documents for the U.K., Bulgaria, France, Italy, Spain, Croatia, Slovenia, Greece, and the Czech Republic.

The Metropolitan police named the three suspects as: Orlin Roussev, 45, Biser Dzambazov, 42, and Katrin Ivanova, 31, who remain in custody. The police declined to comment on whether they were suspected of being Russian spies, according to Reuters. The suspects have not commented publicly on their arrests or the allegations.

According to the BBC's investigation, Roussev has a history of business dealings in Russia. The broadcaster also reports that the suspects had been living in the U.K. for years and had worked a variety of different jobs. 

Dzhambazov and Ivanova moved to the U.K. a decade ago and ran a Bulgarian community organization including familiarizing immigrants with the "culture and norms of British society," according to the BBC.

In 2018, two Russian spies attempted to assassinate former double-agent Sergei Skripal and his daughter in Salisbury. The high-profile assassination attempt used the extremely deadly nerve agent Novichok and while the attempt was unsuccessful, one local woman who happened to come into contact with the bottle holding the nerve agent was killed.