Hashem Abedi, seen in this undated photo, is the younger brother of Manchester Arena suicide bomber Salman Abedi. On Thursday, Hashem Abedi appeared in court and, through his lawyer, denied any involvement in the 2017 attack. Force for Deterrence in Libya/AP hide caption
People stop to observe a minute's silence in St. Ann's Square in central Manchester, England, on Thursday to remember victims of Monday's bombing. BEN STANSALL/AFP/Getty Images hide caption
Forensic teams search the Manchester, England, neighborhood of Salman Abedi, the deceased suspect in Monday's bombing of an Ariana Grande concert. Leon Neal/Getty Images hide caption
A defaced Islamic State flag is emblazoned on a wall in Tikrit, Iraq, in 2015. ISIS will generally claim responsibility for an attack within one day, though it can sometimes take longer. Mohammed Sawaf/AFP/Getty Images hide caption
A police officer stands on duty outside a residential property in Fallowfield, in southern Manchester, on Wednesday as investigations continue into the bombing at the Manchester Arena. Oli Scarff/AFP/Getty Images hide caption
A sign with flowers and candles lay on the ground after a vigil in Albert Square in Manchester, England, on Tuesday, the day after a bombing at an Ariana Grande concert killed at least 22 victims. Kirsty Wigglesworth/AP hide caption
British Prime Minister Theresa May leaves 10 Downing Street after addressing the media in London on Tuesday. Carl Court/Getty Images hide caption
Balloons and flowers filled a memorial in Albert Square in central Manchester, in northwest England, in tribute to the victims of the Monday night attack at the Manchester Arena. Chris J. Ratcliffe/AFP/Getty Images hide caption
A woman lays flowers for the victims of the Manchester Arena attack in central Manchester, England, on Tuesday. Darren Staples/Reuters hide caption
People attend a vigil in Albert Square in Manchester, after the city in northwest England absorbed the worst terror attack on U.K. soil since 2005. Ben Stansall/AFP/Getty Images hide caption