Jim,
Not too difficult if you take the statements of his family and friends into consideration.
They put it down to the racist killing of his friend

Please give details, because no UK news agency has carried such a story.

I could only find this, and it was nearly a month old.
"A source close to Abedi's family told AFP on Friday (5/26/2017) that Abedi's Libyan friend was killed after being stabbed by a group of British youths in Manchester in May 2016. The identity of Abedi's friends was not mentioned further."