The men, who were of Bangladeshi origin but lived in London, were arrested in a vehicle near the Sellafield nuclear site in Cumbria, on Monday afternoon after apparently taking photographs of the site.

The Greater Manchester Police said in a statement that the “Five men arrested under section 41 of the Terrorism Act have today, Wednesday 4 May 2011, been released without charge,”

Section 41 of the British Terrorism Act gives a police officer the right to arrest without a warrant anyone “whom he reasonably suspects to be a terrorist.”

The arrests, made on Monday, came at a time of heightened security globally after the death of al-Qaeda head Osama bin Laden in Pakistan at the hands of American special forces – though the Manchester Police were quick to stress that the detainees were never though to have has al-Qaeda connections.

British Prime Minister David Cameron has said Britain should be extra-vigilant against terrorism in the coming weeks.

Bellona’s nuclear physicist Nils Bøhmer said the police had acted correctly in making the arrests given the sheer wealth of radioactive materials stored at Sellafield, England’s first nuclear military industrial complex site, and later, the site of its first nuclear power plant, Calder Hall.

“There is so much radioactive waste and materials concentrated in such a small area – the highest concentration of waste in the world in such a small space – that it cannot help but be a terrorism concern,” said Bøhmer.

Among the most attractive items for a potential terrorist attack would be Sellafeild’s high-level radioactive waste storage containers and a bunker storing 100 tonnes of plutonium, enough to make several thousand bombs, said Bøhmer.

The arrested men, who are all their 20s, were detained on Monday shortly after 4:30 pm British time, said a statement from the Cumbria Police. The general announcement to the media of the arrests was not made until early evening Tuesday.

The men were held in Carlisle overnight and then moved to Manchester.

Bøhmer, who has visited Sellafield numerous times over the past decade, said security there is tight, and that the Civil Nuclear Constabulary had reacted appropriately.

“Photographs are typically allowed while accompanied on prearranged visits with Sellafield staff, but it is appropriate that unauthorized photography be prohibited against possible terrorist plots,” he said. “It’s common practice.”

Indeed, Bellona has photographed much of the site in reporting trips, and though the photos had to be vetted by staff, they have never been outright censored.