No stone is being left unturned. Forensic investigators in London are looking for clues at the site of the bus bombing which, it is believed, may hold the key to Thursday’s attacks.Other detectives are looking at closed circuit television footage for clues to the identities of the bombers. London police have appealed to the public for help in the search: “We are urgently appealing to anyone who may have photographs, video footage or mobile phone images which were taken either immediately before the attack, at the time of the incidents, or immediately afterwards. These images may contain crucial information which would help detectives in this painstaking and complex inquiry” says one officer leading the inquiry. Mobile phone and Internet companies have been asked to store the content of voice mails, emails and text messages in their systems on Thursday. Britain, meanwhile, remains on high alert in case the bombers strike again. Three men arrested at Heathrow on Sunday were released without charge.