Axel Rudakubana, 18, is accused of killing the girls aged between six and nine at a Taylor Swift dance event in Southport, murders which shocked Britain and provoked days of rioting.
Police said the incident was still not being treated as terrorist-related but said after searches of his home that Rudakubana had now been charged with production of the lethal biological toxin ricin, and the possession of an al Qaeda training manual.
"We have worked extensively with partners to establish that there was a low to very low risk to the public – and I want to make that reassurance clear today," said Merseyside Chief Constable Serena Kennedy.
She said no ricin was found at the scene of the stabbings.
Rudakubana was charged in August but has not yet entered a plea to three counts of murder and 10 counts of attempted murder. He will appear at London's Westminster Magistrates Court on Wednesday (local time) over the two new offences.
Large disturbances broke out in Southport after false reports spread on social media that the suspected killer was a radical Islamist migrant. The disturbances spread across Britain with attacks on mosques and hotels housing asylum seekers.
At the time, police sought to quash the rumours which caused the trouble to erupt, saying the suspect had been born in Britain and there was no terrorism link. Prime Minister Keir Starmer blamed the riots on far-right thuggery.
SPECULATION
Kennedy rejected online speculation that the police were trying to "keep things from the public".
"For a matter to be declared a terrorist incident," she added, "motivation would need to be established. We would strongly advise caution against anyone speculating as to motivation in this case."
Starmer's office said it was important that the police and Crown Prosecution Service (CPS) were allowed to do their jobs and establish the facts.
Made from castor beans, ricin can cause death from exposure to as little as a pinhead amount. Most victims die between 36 and 72 hours after exposure and there is no known antidote.
"Ricin is not infectious between people and the risk to the wider public based on the information available has always been low," Renu Bindra from the UK Health Security Agency (HSA) told reporters.
Bindra said the HSA had been informed by police in August that ricin had been discovered at the suspect's home, prompting leading Conservative opposition lawmakers to express concern that the information had not been made public sooner.
"As more information emerges, it is quite clear there are serious questions to be asked of the police, the CPS and also of Keir Starmer’s response to the whole situation," Kemi Badenoch said on X.
The most famous case of ricin poisoning was in 1978 when dissident Bulgarian writer Georgi Markov was killed when an assassin in London jabbed him with an umbrella that injected a tiny ricin-filled pellet.
