Two activists walked on stage during an event at a downtown hotel Monday, as Harper settled in for a question-and-answer session hosted by the Vancouver Board of Trade.
The pair held up signs attacking Harper's environmental policies before they were quickly removed. Vancouver police have said there won't be any charges.
The security breach has led to questions about how two activists could get so close to the prime minister before his personal RCMP security detail intervened — questions the Prime Minister's Office and the Mounties have so far refused to answer.
The RCMP immediately promised to review what happened and on Wednesday said it had narrowed in the source of the problem, though a spokeswoman declined to elaborate.
"The review of the incident is still ongoing; however, we've identified the mistake and have implemented necessary changes to prevent a repeat occurrence," RCMP Cpl. Lucy Shorey said in an interview from Ottawa.
"We can't get into the details of (the prime minister's) security, and discussing the mistake would, in fact, be discussing security details."
Shorey stressed the prime minister makes hundreds of public appearances every year, mostly without incident.
She also said protecting the prime minister requires the RCMP to weigh several competing interests.
"There is always a delicate balance between the RCMP's duty to protect elected leaders versus the public's right to free speech and the public's access to officials in a democratic society," she said.
Security experts have mirrored those comments, arguing such breaches are inevitable in a society where politicians aren't kept completely isolated from the public.
The incident happened in a crowded hotel ballroom a minute or so after Harper took the stage to field questions from the president of the Vancouver Board of Trade.
Two activists — who have publicly identified themselves as Sean Devlin and Shireen Soofi — walked onto the stage behind Harper and held up their signs. One sign said Climate Justice Now, while the other said Conservatives Take Climate Change Seriously, with a line crossed through the sentence.
Devlin has said he and Soofi used black shirts and aprons they picked up from a thrift store in an attempt to blend in with catering staff. He has described the process as "quite easy" and said he and Soofi weren't approached by security at any point before walking on stage.
Devlin and Soofi were released shortly after the event and spent the rest of the day conducting a series of media interviews.
They are affiliated with Brigette DePape, the former Senate page who was fired after she walked onto the floor of the upper chamber holding a Stop Harper sign during a 2011 throne speech.
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