Newly unsealed documents show investigators found images of nude underage girls were transmitted on an email account they linked to billionaire philanthropist T. Denny Sanford. But the documents unsealed Thursday show investigators couldn’t prove who sent the images or where that person was at the time. Sanford was not charged after South Dakota’s attorney general said its investigation into child pornography found no prosecutable offenses within the state. Sanford’s attorney said in a statement that various people had access to the electronic devices in question and there were also “signs of hacking.” Sanford is the founder of First Premier Bank in South Dakota and the state’s richest man. He has given billions to hospitals, universities and charities.