A radioactive or chemical attack. From [1] SkyNews:

Eight men arrested in an anti-terrorism swoop have been charged with attempting to launch a radioactive or chemical attack.

The suspects were also charged with conspiracy to commit murder and some accused of plotting a terror campaign against major financial landmarks in the USA.

The men were among those seized in a series of raids in London, the Home Counties and in Lancashire two weeks ago.

Dhiren Bharot, from Willesden, north-west London, is charged with possession of plans for the stock exchange and Citigroup bank building in New York, IMF headquarters in Washington DC.

Along with Nadeem Tarmohammed, again from Willesden, he was also accused of owning documents relating to an alleged attack on the Prudential Building in New Jersey.

Police charged Quaisar Shaffi, 25, also from Willesden, with owning part of the Terrorists' Handbook detailing the preparation of explosives.

Among those held is a man described as a senior al Qaeda agent in Britain.