London, Mar 9 (IANS): The investigation into the attempted murder of former Russian spy Sergei Skripal has widened, as police sealed off the graves of his wife and son and confirmed that a total of 21 people had been treated as a result of the incident.

Nick Bailey, a police officer who was exposed to the nerve agent used on the Skripal and his daughter Yulia, 33, remained in a serious but stable condition, reports the Guardian.

Skripal, 66, was convicted of passing secrets to the British intelligence service MI6, but was given refuge in the UK in 2010 as part of a "spy swap".

He and Yulia had visited the Bishop's Mill pub and Zizzi restaurant in Salisbury on Sunday before they were found collapsed on a bench near the Maltings shopping centre.

UK counter-terrorism officers has confirmed that a nerve agent was used in the poisoning and have started investigations to uncover the source.

A source familiar with the investigation told the BBC that it was likely to be rarer than the Sarin gas thought to have been used in Syria and in an attack on the Tokyo subway in 1995.

It was not even VX - the nerve agent used to kill the half brother of the North Korean leader Kim Jong-un in Malaysia last year, the source added.

The use of a nerve toxin, usually only held in state military stockpiles, is being seen as a key indicator of possible Kremlin involvement, the Guardian reported.

On Thursday, the Russian embassy in London sent a tweet, saying of Skripal: "He was actually a British spy, working for MI6."

Moscow has repeatedly denied it has anything to do with the attack

A similar incident took place in 2006 when an FSB officer Alexander Litvinenko was poisoned in with a radioactive cup of tea.

A public inquiry a decade later ruled the Kremlin had ordered the hit.