"The threat is real not only for UK but also to all countries accepting nationals returning from Islamic State front. If they know how to use it (chlorine vehicle-borne improvised explosive devices; chlorine barrel bombs etc.), they will surely find ways to purchase chlorine by legal means not to attract attention," the retired Brigadier General told Sputnik on condition of anonymity.
The source explained that a yellow chlorine cloud would not cause many deaths, but would have a considerable psychological impact.
Countries where the problem of returning jihadists is particularly acute – Britain, France and Denmark – should take strict measures, such as collecting their personal details, according to the source. He added that intelligence services should work to prevent potential attacks and reveal hostile plots and behaviors.
"For a strange reason all jihadists conducting terrorism attacks in Europe (France, UK, Denmark) proved that they were under close surveillance by authorities," the source noted.
Meanwhile, these states aim to re-integrate those who come back from the Middle East into society through rehabilitation programs.
Some of the bombs used by the IS reportedly contained chlorine, which burns the lungs and can kill when inhaled in sufficient quantities.
According to the estimates of US National Counterterrorism Center, the IS group has attracted at least 20,000 foreign fighters in the last three years, with about 500 Britons among them.
