2,611 people have died in terrorist attacks around the world since 2001.
Many of the fatalities were caused by attacks by extremists who had previously been targeted by security services.
Some people have said they did not know about the threat until after they had been targeted.
In the US, President Donald Trump said he had been told by the head of the FBI about the terror threat and had instructed his National Security Adviser Michael Flynn to get on a plane and fly to the UK to “warn people.”
Flynn was asked by a reporter whether Trump had asked Flynn to travel to the United Kingdom to warn people.
Flynn replied that he did not.
The Trump administration has been criticized for the lack of a clear strategy in terms of how to respond to the threat, and the number of people who have died from attacks has soared since the president took office.
While it is not clear whether Flynn is currently in the UK, the White House has not released any official statements about the situation.
Fifty-six people have been killed in the attacks, according to a report published by the Associated Press.