Thornberg attributed the rise primarily to the propaganda machine of the ISIS, which has united different groups of Islamist extremists.
Police have warned some of the victims may never be identified because of the state of the remains.
The talks are taking place three months after Prime Minister Theresa May formally triggered the two-year Brexit timetable.
The incident occurred less than three months since a man drove a car into pedestrians on Westminster Bridge
The Iraqi-born world’s most-wanted man has been rumoured wounded or killed a number of times in the past.
The criminal probe was launched after it emerged that cladding, used on Grenfell Tower is banned in the US.
At least 17 people were killed in the disaster, although that figure is expected to rise, but only 6 have been identified.
Reports released last year said that Clinton destroyed her used phones on at least 2 occasions while serving as Secretary of State.