What does it take to break a terror suspect's GPS tag?

We do not know how the electronic GPS monitoring tag was removed from the ankle of Mohammed Ahmed Mohamed, the terror suspect who went missing after changing into a burka at a London mosque last month.

However, a sturdy pair of scissors was all it took to cut through the strap of an identical tag that was fitted to me the other day.

And there was easily enough room between my ankle and the strap to position the scissor blades before making the cut.

The tag was fitted to me in exactly the same way that all tags are attached to suspects under Terrorism Prevention and Investigation Measures (TPims).

It is made of a type of plastic, which although tough is deliberately breakable enough to be removed in case of an emergency.

"It can be cut and it can be broken off because it has to break at a certain prescribed limit for safety reasons," says Paul, an equipment specialist for the security company G4S, who fitted it to me. The company runs the GPS monitoring contract for government. He requested that his surname should not be used.

"You have got enough flexibility so you can get your sock on and off, you can move it around on your leg for comfort, but you can't get it over the heel and over the ankle to remove it," he says.

"But the main fact is that it cannot be cut off, broken off, or removed in any way without detection and without some evidence of what has caused that to be removed."

Buried inside the length of the strap is a fibre-optic cable that, if interfered with, transmits an alert to G4S's monitoring system.

The strap doesn't have to break or be cut for it to raise the alarm. Simply exerting enough force on the strap to damage the cable sends a signal to the authorities that the device could be being tampered with.

Paul says that on the day Mr Mohamed's tag was removed, the device did what it was designed to do.

"It detected the interference immediately, it transmitted that alert to us and we notified the relevant authorities well within our contracted turnaround time for raising that alarm," he says.

However, by the time the police arrived at the mosque where the tag was removed, he had disappeared.

On the same day he went missing, the Crown Prosecution Service dropped a case it had brought against Mr Mohamed and two other TPim subjects for allegedly tampering with their tags. Had they been found guilty, they could have been given prison sentences lasting up to five years.

However, the CPS said they would not be able to prove to a criminal standard that the tags had been deliberately tampered with.

In cases where the defendants dispute trying to remove or break a tag, expert witnesses are called to see if they can work out what exactly triggered a tamper alert.

Wear and tear

Prof Ross Anderson is an expert in security engineering at the University of Cambridge and he worked on the defence case for the accused TPim subjects.

"It turned out that the terror suspects had been given large ankle tags, which essentially contained a mobile phone," he tells The Report.

"So in effect a strap that had been designed and validated for holding on a curfew tag the size of a wrist watch was being used to hold on something the size and weight of a mobile phone."

Mohammed Ahmed Mohamed is said not to pose a direct threat to the public

Prof Anderson says what had shown up as an attempt to tamper with the tag could be the result of the wear and tear, caused by the strap carrying a heavier device.

But G4S says it is not aware of any false alerts being given by GPS tags and the suggestion the weight of the devices is a factor is wrong.

A spokesman says: "In the limited number of cases where tamper alerts have been raised, and where we have had access to the tag for analysis, we have been able to prove either definite evidence of deliberate interference, or in one case, evidence of damage which would be consistent with falling or catching the device on a hard surface.

"Professional specialist forensic examination has revealed no evidence of any defect, wear and tear, fatigue or fitting error."

The Home Office says the CPS's decision to drop the case against the TPim subjects was not based on the effectiveness of the tags themselves.

The Ministry of Justice says a new generation of GPS tags is due to be introduced in 2014 and is currently subject to robust testing.

Phil Kemp presents The Report on BBC Radio 4 on Thursday 4 December at 20:00 GMT. Follow @BBCNewsMagazine on Twitter and on Facebook