Unfolding just before 10 a.m., while lawmakers were meeting in caucus rooms, the assault rocked Parliament with the boom of gunfire, led MPs to barricade doors with chairs and sent people streaming from the building in fear. Mr. Harper was addressing a caucus when the attack began outside the door, but he safely escaped.

Investigators offered little information about the gunman, identified as 32-year-old criminal Michael Zehaf—Bibeau. But Mr. Harper said, “In the days to come we will learn about him and any accomplices he may have had.”

Canada was already on alert at the time because of a hit-and-run assault Monday against two Canadian soldiers by a man Mr. Harper described as an “ISIL-inspired terrorist.(KH)