Police in Britain say they're investigating a 'network' following the bombing at a pop concert at Manchester Arena.
The father and a younger brother of the suicide bomber, Salman Abedi, have been arrested in Libya - which brings the total number arrested to six.
A woman has also been arrested.
Meanwhile soldiers have moved onto positions around parliament and key government buildings after the country's terror threat level was raised to its highest - critical.
They're replacing armed police who are normally responsible for such tasks, allowing those officers to be deployed elsewhere.
Cressida Dick is London's chief of police.
"There are a number of unknowns since the attack - a huge number of inquiries. The officers in the counter terrorism network near Manchester and beyond are working incredibly fast and whilst we're at 'critical' and there is the possibility of a furthr attack we want to be able to support the public the best we can and to protect them as best we can and we believe we need more officers on the streets."
22 people who lost their lives, 64 people were injured, 20 of them critical - in the attack - several others are still missing.
