Details of the year-long case were released on Wednesday after a British judge lifted reporting restrictions on the trial.
The jury deliberated for more than 83 hours over four weeks.
The Algerian, Kamel Bourgass, 29, was sentenced in June to life in prison for stabbing a policeman to death during a raid in northwest England on 14 January 2003.
He had been on the run since another raid on a London apartment about a week earlier, where officers found recipes for ricin and other poisons.
Bourgass was also convicted of a charge of conspiracy to commit a public nuisance “by the use of poisons and explosives to cause disruption, fear or injury”.
Ricin is derived from the castor bean plant and is one of the world’s deadliest toxins.
It has no known vaccine or antidote and kills cells by preventing them from making proteins.
“This was a hugely serious plot because what it had the potential to do was to cause real panic, fear, disruption and possibly even death to the public”
“This was a hugely serious plot because what it had the potential to do was to cause real panic, fear, disruption and possibly even death to the public,” Peter Clarke, head of Scotland Yard’s Anti-Terrorist Branch, said on Wednesday after news of the verdict was released.
During the raid on 14 January 2003, Bourgass stabbed constable Stephen Oake to death and knifed three other policemen.
Officers testified the raid was botched and the suspects were left unhandcuffed for long periods and intelligence information was bad.
Four other Algerians – Mouloud Sihali, 29; David Aissa Khalef, 33; Sidali Feddag, 20; and Mustapha Taleb, 35 – were cleared of the conspiracy charges on Friday.
Their fingerprints allegedly had been found in the house where the toxin recipes were found.
Prosecutors also dropped charges against four other men – three Algerians and a Libyan – on Wednesday, officials said.
Defence attorney Michel Massih said prosecutors failed to prove conspiracy against the men, alleging that the charges were hastily compiled and stressing that no ricin was ever found.