Mubin Shaikh, the former CSIS informant who was critical to thwarting the Toronto 18 plot, says that after earlier, specific threats, "it's inexcusable that an individual can get into Parliament Hill with a rifle and start shooting people."

Speaking to Evan Solomon Wednesday on CBC News Network, Shaikh said he "couldn't imagine" why security on Parliament Hill wasn't more prepared for an attack.

"It's great for us to think that we are a free, very open society, but we need to get right with the world that we are living in now," he said. 

Shaikh said Wednesday's violence was foreshadowed by the Toronto 18 plot.

"This is exactly what they planned to do: to storm the Parliament buildings, to shoot people," he said.

Since then, Greenpeace was able to climb onto a roof and unfurl a banner on Parliament Hill, he pointed out, and Monday's hit-and-run attack on soldiers in St-Jean-sur-Richelieu should have also been a warning sign.

"Considering that security wasn't even tightened after the threat level being elevated, there are a lot of questions that somebody needs to answer," he said.

"How is it possible that after all those previous indicators that security was not beefed up?"

'Copycat' attack?

Shaikh also told Solomon that he had seen the picture of Wednesday's shooter on an ISIS-affiliated website, something CBC News has not confirmed.

"I'm not saying that there's an official connection, but I can tell you that I saw with my own eyes, that picture posted by an individual who is associated with ISIS."

Shaikh said that ISIS sympathizers are "cheerleading" online, but he doesn't know whether it's being formally directed by ISIS.

"They'll definitely gladly take credit for it, and anyone who's paying attention to Twitter right now, that's exactly what they're doing."

"I think it's just an individual who's acting on his own," Shaikh said, adding that the shooter may have taken "inspiration" from Monday's attack.

"I'm thinking copycat," he said.