British authorities say they believe the 22-year-old bomber was given the explosives used to kill himself and others at the pop concert.
On Thursday, the British Foreign Office announced Rex Tillerson will make his first official visit to to the United Kingdom this Friday "in an expression of UK-US solidarity following the terrorist attack in Manchester earlier this week".
Authorities identified Salman Abedi, 22, as the bomber behind Monday's attack.
"I want to reassure people that the arrests that we have made are significant", Greater Manchester Police chief constable Ian Hopkins said Thursday, "and initial searches of premises have revealed items that we believe are very important to the investigation".
Special Deterrent Force spokesman Ahmed bin Salem said Thursday the mother told investigators Salman Abedi left Libya for England four days before the bombing took place.
A spokesperson for the Libyan authorities said: "His brother felt there was something going on there in Manchester and he thought his brother would do something like bombing or attack".
German police said he made a brief stopover at Duesseldorf airport, while a Turkish official said he had transited through Istanbul airport without saying where he was travelling from.
French Interior Minister Gerard Collomb said Abedi had "likely" been to Syria after the trip to Libya, citing information provided by British intelligence services to their counterparts in Paris.
Attorney General Jeff Sessions said he was already on the case.
Details of his plea for forgiveness from his family have emerged after his mother Samia Tabbal, a nuclear scientist, was detained by police in Tripoli.
UK Prime Minister Theresa May said Wednesday that she planned to confront Trump at a North Atlantic Treaty Organisation meeting about the leaks, which included highly sensitive police photos of the ongoing investigation into the attack.
British officers said the leaks had caused "distress and upset" to the victims' families.
The Trump administration has been plagued by leaks from dissatisfied government employees since he took office, and these latest leaks show how his inability to stop them damages not only USA security but that of our allies.
"This damage is even greater when it involves unauthorized disclosure of potential evidence in the middle of a major counter terrorism investigation".
Britain's terror threat level has been raised to its highest possible level in response to the bombing, amid fears the suicide bomber acted as part of "a network". Armed troops were also sent to guard key sites, a rare sight in mainland Britain.