It's not yet clear what role they may have played in the attack, but police are investigating if bombing suspect Salman Abedi acted alone or if he was part of a larger network.
One woman who was earlier taken into custody has been released.
British police working on the case have resumed intelligence-sharing with USA counterparts after a brief halt because of anger over leaks to US media thought by Britain to be coming from US officials. President Donald Trump has criticized the leaks, calling it "deeply troubling" and demanding the Justice Department to launch an investigation. But the steady stream of information appearing in USA press has persuaded Greater Manchester Police to take the extraordinary step of no longer sharing information on the investigation with the Americans, the BBC reported.
A spokesperson for the Libyan authorities told BBC: "His brother felt there was something going on there in Manchester and he thought his brother would do something like bombing or attack". A Polish couple who had come to collect their daughters after the concert also died, Poland's foreign minister said.
Meanwhile, officials probed possible travel by the alleged bomber, looking for clues to new threats.
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A soldier stands guard at Downing Street, in London, Thursday, May 25, 2017. British authorities were furious.
One device was blown up in a controlled explosion and security sources say there is a real possibility other bombs are yet to be found. Abrini visited Manchester in 2015. He said the bomb's construction suggested a "level of sophistication" that might indicate foreign training.
Another brother, 23-year-old Ismail, was arrested on Tuesday in Manchester.
Abedi is now a manager of the Central Security force in Tripoli. He said his son last visited Libya about six weeks ago and had never gone to Syria. He denied ties to any militant groups or suggestions of extremism.
Trump has yet to fully respond to the news regarding British intelligence. They added they've uncovered items that are believed to be important to the investigation.