Playing devil's advocate here... really, just devil's advocate... but the "authorities" would quickly point out that they did not know, with certainty, that they were only looking for one person. Actually, they still don't know if others are in the area that helped or participated in this attack. The "authorities" would point out that additional explosives were involved and that there was a good possibility of follow-up bombings in or around the same area. They might even argue that by having everyone stay home, they were actually leveraging their ability to find the white-capped bomber more quickly. Homeowners, instead of gawking on the nearest street corner, would be home monitoring their own premises and would be more likely to notice something unusual. In fact, that is exactly how the guy was caught.
In a different time, or a different place, a citizen's posse rushing through the streets to capture a villain and deliver the people's justice seems romantic and apropos. Things have changed... people have changed... and certainly not for the better. The increasing militarization of civilian police forces and the unfettered usurpation of citizen's liberties can be effective - but at what cost? Benjamin Franklin once reminded us, "Those who surrender freedom for security will not have, nor do they deserve, either one." This, I fear, has been forgotten.