It’s really easy. Perrin’s First Law of Terrorism: Terror suspects are ALWAYS known to authorities in advance. Always.
For example: The London Tube Bombing Case: Arrestee No. 1, an 18-year-old “refugee” and foster “child,” was arrested (and released) at the same train station only two weeks ago.
Arrestee No. 2 is a 21-year-old “refugee” and former foster “child.” They say he loves a party.
The first suspect is from Iraq, the second from Syria.
The process goes something like this, overall: Western nation invades or meddles in foreign country. Foreigners are angered by this. Western nation foolishly allows mass migration of angry foreigners into Western nation. Western nation puts up displaced angry (fighting-age male) foreigners in foster homes. Displaced angry fighting-age male foreigner fosterlings bomb local subway train. Repeat process until Western nation is dead.
Meddling there + importing here = suicide.
For only Two Pounds a day you can help a needy child like this hospitalize thirty or so of your countrymen. Facebook/Mirror.