An inquiry into the poisoning of former Russian spy Alexander Litvinenko started at the Royal Courts of Justice today.
The judge leading the investigation said finding out what happened was a matter of “utmost gravity”.
Mr Litvinenko died from radiation poisoning in London in 2006 – nearly three weeks after drinking tea laced with polonium.
Two Russians, Andrei Lugovoi and Dmitry Kovtun, have denied any involvement.
The hearing was told the 43-year-old began working with British security service MI6 in 2003, three years after he first arrived in Britain.
He fled to the UK where he became a vocal critic of the Kremlin.
Jon Naylor explains more.