Rudd also scolded US officials for leaking details about the investigation into the Manchester attack before British authorities were prepared to go public.
British Home Secretary Amber Rudd said Manchester Arena bomber Salman Abedi "likely" had accomplices and she criticized the US government for leaking information about his identity.
"It was more sophisticated than some of the attacks we've seen before, and it seems likely - possible - that he wasn't doing this on his own", Rudd said.
Manchester Mayor Andy Burnham said he had spoken to the United States ambassador about the unauthorised release of Abedi's name, which he said was "not acceptable".
Ramadan Abedi fled Tripoli in 1993 after Moammar Kadafi's security authorities issued an arrest warrant, and he eventually sought political asylum in Britain.
Sixty four people were still hospitalized two days after the blast, Jon Rouse, of the Greater Manchester Health and Social Care Partnership, said.
Police are trying to establish if bomber Salman Abedi acted alone or whether there could be a risk of further attacks.
French Interior Minister Gerard Collomb said he had "likely" been to Syria, citing information provided by British intelligence services to their counterparts in Paris.More news: Rory McIlroy pulls out of BMW PGA Championship due to injury
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He told French television the suspect "grew up in Britain and then suddenly, after a trip to Libya and then likely to Syria, became radicalised and made a decision to carry out this attack".
When asked whether Abedi was part of the larger network, Collomb said: "We don't know yet, but perhaps yes. In any case, [his] links with ISIS are proven".
The school said Olivia, reportedly 15, had been with a friend who has since undergone surgery to treat injuries sustained in the explosion.
"The British police have been very clear that they want to control the flow of information in order to protect operational integrity, the element of surprise, so it is irritating if it gets released from other sources", Rudd told BBC Radio on Wednesday, when asked about US leaks.
The minister said she was "not surprised at all" that the Islamic State group had claimed the attack but said there was no information yet to confirm the extremist organization's active direction.
Prime Minister Theresa May placed the country on its highest level of terror alert - "critical" - for the first time since June 2007, when it was sparked by an attack on Glasgow airport.
Pressed on whether the Americans had compromised the investigation, she said: "I wouldn't go that far, but I can say they are perfectly clear about the situation and that it shouldn't happen again".
The UK Independence Party was the first party to announce it would resume campaigning.