The Manchester bomber Salman Abedi is believed to have travelled to Syria and had "proven" links with Islamic State, France's interior minister has said.
Gerard Collomb told French television that both British and French intelligence services had information that Abedi had been in Syria.
The disclosure came as Home Secretary Amber Rudd confirmed the UK security services had been aware of the British-born 22-year-old.
"We do know that he was known up to a point to the intelligence services," she told Sky News.
"I am sure that we will get more information about him over the next few days and the next few weeks."
Salman Abedi perpetrated the worst outrage Britain has seen in a decade just days after returning from Libya, according to reports.
The 22-year-old's visit to his family's native country fuelled concerns he was preparing for Monday's deadly assault under the guidance of hardened jihadists.
The Times reported the Manchester-born bomber spent three weeks in the war-torn north African nation before the attack on Manchester Arena, in which he was killed.
A friend told the paper: "He went to Libya three weeks ago and came back recently, like days ago."
Both Islamic State (IS), who claimed responsibility for the atrocity, and al Qaida have a presence in Libya, but the possibility remains that Abedi travelled to their heartland in Syria for training.
Prime Minister Theresa May said on Tuesday night it was possible he had planned his deadly attack with a "wider group of individuals".
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