I haven't researched the subject in years, but perhaps you could start with the death of Henry Marshall at the USDA? The information he was gathering was related to the business schemes of Estes, Johnson, Clint Murchison (Sr), and others. (Estes was eventually sent to federal prison over related charges, but the investigation basically stopped there.) From Wikipedia:
"In 1962, after information came to light that Estes had paid off four Agriculture officials for grain storage contracts, President John F. Kennedy ordered the Justice Department and FBI to open investigations into Estes' activities and determine if Secretary of Agriculture Orville L. Freeman had also been compromised (Freeman was cleared). Congress conducted hearings on Estes' business dealings, including some that led to Vice President Johnson, a long-time associate of Estes."
