Ottawa was under attack on Wednesday after a man with a rifle killed a soldier standing guard at the National War Memorial near the prime minister's office, before seizing a car and driving to the front entrance of Parliament Hill.

Cpl. Nathan Cirillo, 24, a reservist from Hamilton, died a short time after the attack despite frantic efforts to revive him at the base of the Tomb of the Unknown Soldier.

Michael Zehaf-Bibeau, the suspected gunman who was later shot dead, had a criminal record in B.C. and Quebec.

While it will no doubt take some time to piece together what led to Wednesday's events in Ottawa, here are five pointed questions that need to be answered.

1. Open House

After shooting the soldier at the National War Memorial, the gunman jumped into a car parked on Wellington, steps away from the Prime Minister's Office at the Langevin Block, which is across the street from the Parliament Buildings. He then made his way to Parliament Hill, where he abandoned his car in front of the Parliament Buildings on Wellington St.

Once inside the Parliament Hill lawn, he hijacked a ministerial vehicle and drove towards the building. There, he walked right inside, where Conservative and New Democrats were holding their weekly caucus meetings. Prime Minister Stephen Harper was in one of those rooms.

How did the gunman get into Centre Block with a rifle?

2. Accomplices?

The gunman was shot and killed on Parliament Hill, but people also reported gunfire near the Chateau Laurier Hotel and then from the Rideau Centre shopping mall, both steps away from where the gunman first parked his car across from the National War Memorial.

What led authorities to believe there was more than one gunman?

3. Police preparedness

Police said the shooting caught them by "surprise" but earlier this week, another soldier died in St-Jean-sur-Richelieu, Que., after Martin (Ahmad) Couture-Rouleau, a man police describe as "radicalized," rammed his car into two soldiers killing one of them.

Did police in Ottawa take any safety measures following the incident south-east of Montreal?

4. Threat level

Canada's threat level had recently been elevated from low to medium, but officials said there was no intelligence on any imminent threat.

What additional precautionary measures were taken after the threat went up?​ 

5. Tracking 90 Canadians 

The RCMP has said it is investigating 90 Canadians as part of 63 current national security investigations.

Was the gunman in Ottawa on the radar of Canadian authorities?