The scandal over claims Britain\'s top-selling Sunday tabloid hacked the phone of a murdered teenager took a new turn as further allegations emerged and the prime minister called for a \"vigorous\" investigation. It emerged late Tuesday that the parents of the victims in another prominent murder case, schoolgirls Holly Wells and Jessica Chapman, have been contacted by detectives investigating phone hacking at the News of the World. Prime Minister David Cameron said police should pursue their investigation into the claims in \"the most vigorous way\". He said police \"should feel they should investigate this without any fear, without any favour, without any worry about where the evidence should lead them. They should pursue this in the most vigorous way that they can\". \"If they are true, this is a truly dreadful act and a truly dreadful situation. What I\'ve read in the papers is quite, quite shocking,\" Cameron told a press conference during a visit to Afghanistan. News International chief Rebekah Brooks, who was editor of the News of the World at the time of schoolgirl Milly Dowler\'s disappearance in 2002, told staff that the allegations were \"sickening\" and \"almost too horrific to believe\". \"If the allegations are proved to be true then I can promise the strongest possible action will be taken,\" said Brooks. In a further twist the prime minister was dragged further into the crisis when News International revealed late Tuesday that it had forwarded evidence to the police which alleged former editor Andy Coulson had authorized payments to police in return for information. Coulson, who was editor between 2003 and 2007, also worked as Cameron\'s director of communications from 2007 to 2011 when he resigned as allegations emerged, although he has always denied any wrongdoing. The tabloid has been dogged by claims of phone hacking ever since its royal editor and private investigator Glenn Mulcaire were jailed for the practice in 2007, and a new investigation was launched in January amid a wave of fresh allegations. Dowler disappeared aged 13 on her way home from school near London, sparking a high-profile search. Her bones were found six months later, although it was not until last month that her killer Levi Bellfield was convicted. According to the Guardian, private investigators and journalists listened to increasingly desperate messages on Dowler\'s phone left by her parents and friends as the days went by without any word from her. Related article: UK media shamed by \'squalid\' phone hacks claims When her voicemail box became full, they even deleted several messages to make room for new ones -- an action that her loved ones and police mistakenly took as proof that Dowler was still alive and using her phone, the report said. Police refused to comment on the allegations about Dowler, but Brooks said News International would cooperate fully with police, as well as conducting its own internal inquiries. The Dowlers\' lawyer, Mark Lewis, said they would sue the tabloid for its \"despicable\" behaviour and called for Brooks to quit. Brooks maintained it was \"inconceivable\" that she had known about the actions and said she would remain in her position. In the first sign of a commercial backlash from the allegations, carmaker Ford announced that it was pulling its advertising from the News of the World. In a rare moment of reflection, The Times newspaper, also owned by Rupert Murdoch\'s News International group, said in an editorial that if the claims were true then \"there will not be a journalist in the country who, after the warranted anger, will not feel shamed and depressed.\"