Footage has emerged which appears to show a truck, apparently driven by Abdul Waheed Majeed on his way to carry out a suicide attack in Aleppo.
Majeed is said to have driven a lorry into a jail in Aleppo and detonated a bomb last week, and is thought to be the first Briton to stage a suicide attack in the war-torn state.
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EnQuest said the evacuation was taking place as a precautionary measure.
Opioid use has been linked to the deaths of more than 400,000 people over the past two decades in America.
The 21-year-old man was arrested on Monday night on suspicion of two counts of murder after the incident in Milton Keynes.