How did Mr. Big rise to prominence in the novel, Live and Let Die?

Upon moving to America, Mr. Big worked for a gang high-jacking trucks. He saved his money and eventually moved to Harlem, investing in a string of nightclubs and brothels. During World War II, Mr. Big worked for the American Office of the Secret Services. After the war, he disappeared and it is assumed he was being trained in Russia. He resurfaced in Harlem five years later and was quickly suspected by the FBI to be a Soviet spy.