Early in 1968, the Krays employed Alan Bruce Cooper who sent Paul Elvey to Glasgow to buy explosives for a car bomb. Elvey was the radio engineer who put Radio Sutch on the air in 1964, later renamed Radio City. After police detained him in Scotland, he confessed to being involved in three murder attempts. The evidence was weakened by Cooper, who claimed that he was an agent for the US Treasury Department investigating links between the American Mafia and the Kray gang. The botched murders were his attempt to put the blame on the Krays. Cooper was being employed as a source by one of Read's superior officers, and Read tried using him as a trap for the Krays, but they avoided him.
Eventually, Scotland Yard decided to arrest the Krays on the evidence already collected, in the hope that other witnesses would be forthcoming once the Krays were in custody. On 8 May 1968, the Krays and 15 other members of the Firm were arrested. Exceptional circumstances were put in place so as to stop any possible co-operation between any of the accused. Nipper Read then secretly interviewed each of the arrested, and offered each member of the Firm a deal if they testified against the others. Whilst in prison, the Krays had come up with a plan, which included having Scotch Jack Dickson to confess to the murder of Cornell, Ronnie Hart to take the McVitie murder and Albert Donoghue to stand for Mitchell.
A letter Ronnie sent to his mother Violet from prison in 1968 also refers to Monica: "if they let me see Monica and put me with Reg, I could not ask for more." He went on to say, with spelling mistakes, "Monica is the only girl I have liked in my life. She is a luvely [sic] little person as you know. When you see her, tell her I am in luve [sic] with her more than ever." Ronnie subsequently married twice, marrying Elaine Mildener in 1985 at Broadmoor chapel before the couple divorced in 1989, following which he married Kate Howard, whom he divorced in 1994.