His friend, also of Libyan descent, died after being stabbed by British youths in Manchester in May 2016, the source said on condition of anonymity.
"That incident stirred up a sense of anger among young Libyans in Manchester and especially Salman, who clearly expressed his desire for revenge," he said.
"We were able to calm the young people in the neighbourhoods who felt they were targeted... as Muslims," he said. "But it seems that Salman did not forget the incident."
"I personally talked with him and tried to convince him that it was just a criminal act," he added.
British media reported that Abdul Wahab Hafidah died after being run over and stabbed in the neck in Manchester's Moss Side district in May last year.
His suspected killers are still on trial.
Monday's attack at a pop concert killed 22 people and was claimed by ISIS.