Five British men have been convicted and sentenced to life in prison for their roles in plotting terror attacks on nightclubs, shopping centers and trains in the United Kingdom. The investigation, known as "Operation Crevice," was one of the largest counterterrorism operations ever seen in the U.K. "The threat posed by this group of men demanded an unprecedented response from the police and Security Service," said Peter Clarke of the Metropolitan Police. "It was imperative that the public were protected from these men and their plans." Most of the men had attended a terrorist training camp in Pakistan where they were taught how to make explosives, police say. In 2004, the men were picked up in a series of raids where police seized more than 1,000 pounds of ammonium nitrate fertilizer. Clarke said that once the bomb-making materials were purchased and the men were taking targets, the police decided to move in. Click Here for Full Blotter Coverage. "In this case we decided to arrest the plotters when we feared that they were getting closer to mounting their attack," said Clarke. "They had to be stopped." A life sentence in the U.K. does not necessarily mean the men will be behind bars for the rest of their lives. Three of the men — Omar Khyam, Anthony Garcia and Waheed Mahmood — will all serve a minimum of 20 years before they can be considered for parole. Jawad Akbar and Salahuddin Amin will each serve a minimum of 17-and-a-half years. At the sentencing, the judge described the suspects as "cruel and ruthless men."