The possibility of terrorists crossing the U.S. northern border with Canada is much more worrisome than the southern border with Mexico, according to former CIA deputy director Mike Morell. Morell, who twice served as acting director of the CIA in the Obama administration, was interviewed on CBS This Morning on Thursday, the day after a newly-converted Muslim shot and killed a Canadian soldier in the country's parliament building.

Host Charlie Rose asked Morell what he "makes" of the terrorist action:

Morell: When ISIS first came into our consciousness, you’ll remember that we talked on this show about near-term threats. One was ISIS sending fighters who had gone to Syria back here to conduct attacks. The other was folks here who ISIS would be able to radicalize with their message. I think that's what we saw yesterday in Canada. That's certainly what we saw in Canada three days ago. And just yesterday in Britain, the British arrested a woman planning attacks who had been radicalized by ISIS. So we're starting to see one of the two threats become a reality here, Charlie.

Rose: So clearly home-grown could be an issue in the United States, but what about radicalized Canadians coming across the border. How easy would that be for them?

Morell: I'm much more concerned about the Canada border, Charlie, than I am the Mexico border because it's much easier to come across the border. To stop somebody who's been radicalized in Canada from coming across that border requires that you know about them, that the Canadians know about them and tell us and that they try to cross legally. And you know there's many, many ways to cross the border illegally. So I worry about that.