Parliamentary systems and procedures will be examined to identify any needed improvements, Scheer said Thursday.
Michael Zehaf Bibeau was fatally shot in the Parliament Buildings on Wednesday after killing a soldier at the nearby National War Memorial — an incident that shook Ottawa and resulted in a lockdown of the downtown core for much of the day.
"Members will ask — indeed Canadians will ask — how this came to occur and what specifically will be done to prevent future occurrences," Scheer said. "These are legitimate questions and they require comprehensive answers."
Scheer said he would work with all members of Parliament "to ensure that the House obtains answers to these vital and important questions."
Parliament was closed to visitors Thursday, and tours were cancelled, he noted.
"However, I have stressed that these must be temporary measures," Scheer said to applause from his fellow MPs. "Parliament must remain an institution that is both open and secure."
A national security expert said he expects there will be permanent steps to tighten parliamentary security.
"The Hill will become a less friendly and open place," said Wesley Wark, a historian who teaches at the University of Ottawa.
RCMP Commissioner Bob Paulson also said Thursday that the Mounties would now provide round-the-clock personal security for the prime minister in the wake of the siege, which saw Stephen Harper spend several minutes holed up inside the Conservative caucus room, unguarded.
Surveillance video of Zehaf Bibeau's quick entry to the Hill on foot and subsequent carjacking of a vehicle that he drove to the Centre Block will help the RCMP determine "the measures that we need to take now to make the parliamentary precinct more secure," Paulson told a news conference.
"The security picture on Parliament Hill needs to be evaluated."
During question period, NDP Leader Tom Mulcair sought assurances on behalf of Canadians that the parliamentary grounds would "remain an open place for them."
Harper said the government has traditionally ensured a balance between security and freedom.
The prime minister added the government was looking at options under the law to "strengthen the ability to surveil, detain and arrest individuals" who pose threats.
"I know individual security agencies will also be looking very carefully at the events of yesterday to determine what else has to be done."
The Conservatives were already poised to introduce changes to the law governing Canada's spy service before the shooting.
The proposed amendments would give the Canadian Security Intelligence Service more power to track terror suspects abroad and provide blanket identity protection for the agency's human sources.
Prime Minister Stephen Harper told the Commons the changes would be "expedited" but there was no indication as to when a bill would be introduced.
The federal government should reflect calmly on the shooting incident before changing counter-terrorism laws, Wark said.
The key question is whether the security laws passed after the 9-11 terrorist attacks have served Canada well, he said. Only once there are answers should the government consider changes to resources, organizations or laws.
"Otherwise we are just flailing in panic," Wark said. "I would like to think that is unCanadian."
For now, the RCMP is sitting down with CSIS to re-evaluate a list of 93 suspected radicals deemed to pose "the greatest risk" in order to arrange surveillance or gather evidence to make arrests, Paulson said.
"We have not made arrests today, we don't have any intentions of making imminent arrests," he said.
"And as the evidence is collected and the cases are built, we will be making arrests."
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