OTTAWA – On the fourth anniversary of the Parliament Hill terrorist attack, Prime Minister Justin Trudeau is encouraging people to "show gratitude to the valiant Canadians in uniform, past and present."

It was four years ago Monday, that Michael Zehaf-Bibeau gunned down Cpl. Nathan Cirillo, the reservist who was standing guard at the National War Memorial.

The attacker then made his way to Centre Block, where he was shot and killed in an exchange of gunfire in the Hall of Honour, as MPs hid in rooms on either side.

Two days earlier, Warrant Officer Patrice Vincent was killed in a separate attack in Saint-Jean-sur-Richelieu, Que.

In a statement, Trudeau called it a "sombre anniversary."

"As we pay tribute to these fallen members of the Canadian Armed Forces, we also pay our respects to the brave first responders and members of our Parliamentary family who acted without hesitation to keep us safe," Trudeau said.

Martin Magnan, a man who tried to help save Cpl. Cirillo’s life that day told CTV News Channel that he is still dealing with the emotional repercussions.

"These men and women in uniform – they die. They really do die for us," Magnan said.

In a Facebook post, Conservative Leader Andrew Scheer described the Hill attacker as "an attacker who then tried to strike at the very heart of our democracy."

He said, in honouring Cpl. Cirillo, "we commit ourselves again to our fight to rid the world of terrorism, and express our gratitude to the men and women of the Canadian Armed Forces who stand on guard for us with pride."