Police in Stockholm were investigating on Monday whether the gunman was a policeman.
Thousands of disappointed fans were evacuated from the sphere-shaped arena. In one video uploaded in YouTube, fans were chanting “Why?” when they were asked to leave.
Swedish media reportage suggested the incident the breach was the result of an off-duty police officer being allowed to enter the venue carrying a gun.
Under normal circumstances, off-duty police officers are not allowed to carry weapons under normal circumstances.
The concert has been rescheduled for Tuesday, ticket-holders who will not be able to attend will be reimbursed.
In a statement on U2’s website the organisers said:
“The organizers apologize for the inconvenience caused and thank the audience for their understanding. Their safety is the priority,”