The FBI and US Justice Department are helping the UK police to investigate Southport killer Axel Rudakubana.
The 18-year-old was jailed for a minimum of 52 years in January after murdering three girls at a Taylor Swift-themed dance class in July last year.
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.
“A specialist liaison CPS Prosecutor in the United States has been working with international partners to obtain material which may be relevant.
“We are thankful to the US Department of Justice and the FBI for their ongoing assistance and the importance which they have placed upon our request.”
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Southport murders: What happened?
According to reports, investigators now hope to recover deleted searches from the killer’s Google and Microsoft accounts within weeks.
Rudakubana pleaded guilty to murdering Alice da Silva Aguiar, nine, Bebe King, six, and Elsie Dot Stancombe, seven.
He also admitted trying to murder eight other children, instructor Leanne Lucas and businessman John Hayes, as well as charges of producing ricin and possession of information useful for the purposes of terrorism.
Rudakubana will be 70 before a parole board can consider releasing him, but the judge, Mr Justice Goose, said it was “highly likely” he will never be freed.
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.