What is ricin the biological substance police say they found at ...

The teenager accused of stabbing three girls to death in Southport now faces two further charges.

Axel Rudakubana, 18, from Lancashire, faces one charge of the "production of a biological toxin, namely ricin, contrary to Section 1 of the Biological Weapons Act 1974".

He also faces a terror charge of possession of information "likely to be useful to a person committing or preparing an act of terrorism, contrary to Section 58 of the Terrorism Act 2000".

This came after police said they found ricin and an al Qaeda training manual, during a search of his home.

He had already been charged with three counts of murder and 10 counts of attempted murder following the mass stabbing at a Taylor Swift-themed dance workshop in July.

What is ricin?

Ricin is found naturally in castor beans.

When the beans are processed to make castor oil, it leaves a mash that contains the toxin.

It's classed as a biological substance because of its origin, but it doesn't spread from person to person like a virus or bacteria.

But like a chemical, it can cause cross-contamination, for example through contact on clothing.

The UK Health Security Agency says there is no evidence that any member of the public, or the emergency services, was exposed to the ricin allegedly found at Rudakubana's home.

Anybody who had been exposed would have developed the first symptoms within 24 hours or so.

If it's inhaled it causes progressive breathing difficulties, ultimately causing respiratory failure and death.

If it's ingested it initially causes diarrhoea and vomiting that can be severe enough to result in dehydration.

Days later the body's organs begin to shut down, possibly causing death.

Skin contact causes irritation, but isn't itself serious. The risk is that people who pick it up on their fingers, for example, then ingest it.

Roughly 2-6% of cases of ingestion result in death.

The fatality rate from inhalation is unknown.

No information has been given by police on what form the ricin was found in.

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But in previous cases, it has been processed as a powder, mist, pellet or dissolved in water or acid.

In 1978, the Bulgarian dissident Georgi Markov was assassinated in London by an operative, thought to be from the Bulgarian secret service, who used an umbrella to inject a pellet of ricin under his skin.

In 2013, letters tainted with ricin were sent to President Obama, the New York mayor, and others.

On that occasion, nobody came to harm.