Task: Question Answering
Input (English): Passage: Following the cover up of a crash of a nuclear bomber at an American Air Force base in the UK, Dennis Markham, a prominent Member of Parliament and opponent of the American nuclear presence in the United Kingdom, is planning to ask questions about it in Parliament. Before he can, he is reported by a London paper to have been seen leaving a woman's home on the same evening as she is visited by a military attache from East Germany, Markham's loyalty to his country is questioned. He is hounded by the media and is forced to resign.
The author of the newspaper exposé, Nick Mullen, continues his work alongside colleague Vernon Bayliss who suspects that Markham was framed. When Bayliss dies from a supposed heart attack the same night as Bayliss' flat is ransacked by someone who was not after money or valuables, Mullen suspects something deeper at work. He then finds some evidence of the cover-up of a near-accident at a nuclear site and a secret US Air Force base. With the help of Markham's secretary, Nina Beckman, Mullen continues to investigate the affair despite a break in of his flat, surveillance and other attempts of the British Government to stop him. In the end, Mullen and Beckman are seemingly killed in an explosion, but Mullen's story about the cover-up is published. 
Question: Who tried to stop Nick Mullen of his investigation?
Output (English): the British Government.