Five men were arrested under anti-terrorism laws near a nuclear waste-processing plant in northwest England, police said on Tuesday.
Local police said the men were detained under the Terrorism Act near the Sellafield plant on Monday after officers stopped a car. All are from London and aged in their 20s. They were being held a police station in Manchester.
Four houses in east London were raided Tuesday in connection with the arrests, police told the Guardian newspaper. Counterterrorism officials were questioning the men.
Andy Hayman, an NBC News security analyst who formerly served as the U.K.'s most senior counter-terrorism officer, said sources told him the five men appeared to be South Asian in origin and had been taking "suspicious" pictures of the plant's perimeter fence. Police would not confirm that report.
The men were not previously on any watch list or under surveillance, the sources told Hayman.
Sellafield, about 250 miles northwest of London, was the site of Britain's first nuclear power plant, but now only reprocesses nuclear waste.
Prime Minister David Cameron has said Britain should be extra-vigilant against terrorism in the coming weeks following the death of Osama bin Laden.
Greater Manchester Police said the investigation was in its early stages, but the force was "not aware of any connection to recent events in Pakistan."