London train bombing: 5 things to know

Here are five things to know about the bombing at the London Underground train station at Parsons Green on Friday, which caused 30 injuries. Police in Kent arrested an 18-year-old man Saturday in the port area of Dover and are holding him for questioning under the Terrorism Act. He has not been charged or identified.

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  • This is the fourth time the United Kingdom's national terror threat level has been raised to "critical" since the system was made public in 2006. The last time was in May after the Manchester Arena bombing.
  • Mark Rowley, the London Metropolitan Police's assistant commissioner, said a suspected improvised explosive device (IED) was thought to be responsible for the blast.   
  • Police sources said the device only partially exploded. Initial examination by explosives experts led them to conclude it was "viable," meaning the device was meant to explode more fully. A circuit board was recovered from the scene.
  • Three of at least 30 people injured remained in a London hospital Saturday.
  • ISIS has claimed involvement in the attack without any evidence to support that claim.  The organization said via its Amaq news agency that a "detachment" from the group had carried out the attack.

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