The Netwalker criminal gang attacked University of California San Francisco (UCSF) on 1 June.

IT staff unplugged computers in a race to stop the malware spreading.

And an anonymous tip-off enabled BBC News to follow the ransom negotiations in a live chat on the dark web.

Cyber-security experts say these sorts of negotiations are now happening all over the world - sometimes for even larger sums - against the advice of law-enforcement agencies, including the FBI, Europol and the UK's National Cyber Security Centre.

Netwalker alone has been linked to at least two other ransomware attacks on universities in the past two months.

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BBC NEWS