
The first time: The night of April 18th-19th, 1775
The second: The night of April 18th-19th, 2013.
Yesterday, it was due to a manhunt for the "Boston Marathon bombers". Everything was "locked down", a euphemism for martial law. (The euphemism is arguably scarier than the term it replaces, in this case). In 1775, it was amidst a wild political climate which saw Paul Revere ride through towns of Massachusetts shouting "the British are coming!" (forevermore to be learned-about by American elementary school students). Fighting followed.

One of the riders passed right through Cambridge.
Cambridge, it seems, was the place of residence of the "Boston bombers". On April 19th, 2013, one was killed, and one captured, in adjacent "Watertown".
Actually, there is a pretty amazing synchronicity:
- Revere was captured about 1 AM on April 19, 1775.
- The older Boston bomber, Tamerlan Tsarnaev, was also located by authorities at about 1 AM on April 19th (of 2013). (Tsarnaev was killed, Revere was not).

at about 1 AM EDT, April 19, 2013 /
238 years to the day and hour
after Paul Revere was captured
A Korean I spoke with on the matter pointed out that it was just three dead. (Three "civilians" and, later, one dead policeman). Bad, yes, but hardly necessitating a mass panic. The radio correspondents I was listening to were in a total daze: They said things like "we've never seen anything like this!" All regular programming was cut for round-the-clock coverage. Everything was closed. Millions were under martial-law: One million people within a certain radius were ordered indoors under threat of force (true martial-law), and millions more in Greater Boston were intimidated into staying off the street (defacto martial-law) too. Photos of deserted streets in Boston emerged. An entire city, empty. This was astonishing, to me.
Was this level of hysteria an overreaction? I think so, but it is also understandable: This kind of drama builds esprit-de-corps by giving people a shared and memorable experience. That is highly important, and something lacking in the USA in recent....decades, it seems to me.