With recent events across the country people are talking more about the terror watch list - but does it actually exist?

"There are different types of list," explains Margaret Gilmore, an expert in counter-terrorism at the Royal United Services Institute (RUSI).

"The behaviours watch list can affect anybody and everybody."

She says it looks out for specific behaviour that is considered suspicious.

"For example there was a time when young people, especially in their teens, going to Syria were being monitored.

"The police were very, very worried that young people had been groomed into thinking they would have a good life if they went out to Syria and worked for ISIS.

"So anybody who was young and travelling out via Turkey, on those flight routes going into Istanbul on their own, would be stopped."

But there is a list of specific names too

Margaret says that even this list is split into two.

"There are names that are down as extremists and we've been told there are more than 20,000 of those in the UK, but they are not watched all the time.

"If they did anything out of the ordinary - if they weren't in their normal location, if they were acting in an odd way and it was brought to somebody's attention, or they did something criminal - then they would be looked at doubly."

Then there's the people MI5 are 'very concerned about'

According to Margaret there are around 3,000 names on this list, and MI5 are very concerned about those people.

"We know that those people will be under pretty regular surveillance and watched a lot of the time."

'Everybody has their own list'

"The UK has its own list but they share the information on it with trusted partners in other countries."

Margaret says this is to find links between names.

"They are looking for networks - so it's, 'Here's his name, what's your list showing?'.

"And it might show that he had been to the United States, so perhaps was a threat to America, or maybe it would help the British lead to somebody else who was pulling the strings in Syria.

"As well as having their own lists, there are also European lists and international ones too."

So what happens when you're on a list?

"Security has a very clear list of people they believe have been to Syria to work with ISIS, so when they come back in to this country on a British passport they are stopped," says Margaret.

"They have to be let back in because you can't just send somebody off if they are a British resident.

"But they will be interviewed, asked if they are sorry and asked if they want to go on a prevent scheme.

"And they will be told they will probably be monitored for a while.

"They may even be put on a tag and have restrictions put on them if the authorities believe that to be necessary."

And Margaret says that sometimes, if someone has been looking at the "wrong website" they may have their passport confiscated to prevent them from travelling to Syria in the first place.

Why not just arrest everyone on the list?

Margaret says that sometimes they do just that.

"If they are aware that there are people on the list who are near something big then they may actually arrest them.

"But if you don't have evidence, you can't charge," says Margaret.

"I remember before the 2012 Olympics a number of people were arrested on suspicion of terrorism and then released without charge.

"And that was simply a way for the police to say, 'We're watching you, we absolutely have you clocked, don't think of trying anything'."

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