Bulgarian authorities are analysing DNA, fingerprints and the Michegan drivers licence used by the man who blew up a bus full of Israeli tourists in an effort to identify him, the Associated Press reports.
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It comes after security was stepped up around the country following the Manchester Arena attack which killed 22 people.
A murderer nicknamed 'Houdini' after escaping the death penalty seven times has been given the lethal injection.
The distress signal sparked a police hunt in the area to find the person or group who made it.