The FBI and US Justice Department are helping the UK police to investigate Southport killer Axel Rudakubana.
In a joint statement, Merseyside Police and the Crown Prosecution Service (CPS) said on Saturday: "The Southport attack has devastated the lives of the victims, their families and the wider community.
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The attack sparked the UK riots last summer after false claims online that Rudakubana - born in Cardiff to Christian parents from Rwanda - was a newly arrived asylum seeker.
The government has announced an inquiry into how the state failed to recognise the risk posed by the killer and Prime Minister Sir Keir Starmer has said he will consider changing the definition of terrorism if necessary.
