T. Denny Sanford, the richest man in South Dakota and a major donor to children's charities, was being investigated for possible possession of child pornography, according to four people familiar with the probe.

Investigators with the South Dakota attorney general's Division of Criminal Investigation obtained a search warrant as part of the probe, according to two of the people, who spoke on the condition of anonymity. They said the case was referred to the Department of Justice for further investigation.

Sanford has not been charged with any crime. It's not clear what evidence, if any, investigators used to obtain the search warrant or what they collected from the search. It's also not clear how the investigation may have progressed since federal authorities received the referral or if it is still ongoing.

Sanford's lawyer did not respond to requests for comment, but on Friday night he told the Argus Leader, a South Dakota newspaper, “Although we know very little about any state or federal inquiry relating to Mr. Sanford, we do know those authorities responsible for investigating allegations obviously did not find information or evidence that supported or resulted in any criminal charges."

Sanford controls First Premier Bank, a major issuer of high-interest credit cards for people who have poor credit. Worth an estimated $1.6 billion, he is a major donor to the state's political figures, including its U.S. senators and state-level officials, and a prolific supporter of children's organizations and other charitable causes, including a major hospital system based in South Dakota that bears his name.

Sanford has close ties to top state and federal political figures, including Sen. John Thune, whose wife has worked for a Sanford-affiliated organization, and the state's former attorney general and senate majority leader, both of whom now work for Sanford.

He was recently named one of the country's top 10 philanthropists in 2019, clocking in just behind Bill and Melinda Gates. He also donates the Children's Home Society of South Dakota, which serves young victims of violence and abuse, and children's hospitals. Academic and research institutions across the country are named after him.

The South Dakota attorney general's chief of staff, Timothy Bormann, said the office's policy is to neither confirm nor deny the existence or details of any criminal investigation.

A Justice Department spokesman did not immediately respond to questions from ProPublica.

Sanford is being represented by Marty Jackley, a former attorney general and U.S. attorney in South Dakota. As part of an unsuccessful primary campaign for governor, Jackley highlighted his role as U.S. attorney in creating a stand-alone Internet Crimes Against Children Task Force in the state. As attorney general, he co-chaired a national campaign to rescue children from sexual exploitation.

Sanford is a towering presence in South Dakota's largest city, Sioux Falls, sometimes jokingly called Sanford Falls. The 12,000-seat indoors sports and events arena bears his name, as do several of the largest buildings in town. Outside the castle-like Sanford Children's Hospital, a statue named “For the Love of Children" depicts Sanford kneeling beside two small boys and a girl. Another statue, “Chasing Dreams," at the Sanford Sports Complex portrays children running toward him with basketballs.

“My primary bent, in terms of philanthropy, is directed at small children, to give them the opportunity to realize a full life," he told the Argus Leader in 2004.

Websites for Sanford organizations show the billionaire frequently interacting with kids involved in his charitable programs, visiting a small girl in a hospital bed, smiling at drawings with a schoolchild sitting in his lap, and passing out candy canes to a group of kids. A newsletter for the Children's Home Society of South Dakota, which received a record $55 million from Sanford in 2019, featured an illustration of him drawn by a child in appreciation of his support.

According to his foundation biography, Sanford has given almost $1 billion to Sanford Health, the nation's largest rural nonprofit health care system, including a $400 million gift in 2007 that was called the largest contribution ever to an American health care organization. After a $12.5 million gift this month from Sanford, the University of South Dakota's law school named itself after Sanford's friend and lawyer, David Knudson, the former state Senate majority leader.

Sanford has close ties to state and federal elected officials. He has spent more than $425,000 on political contributions, according to databases that track election spending, mostly to Republican causes and candidates including small sums to Donald Trump.

Affiliates of the health system have employed Kimberley Thune, the wife of the second-ranking Republican in the Senate, according to John Thune's financial disclosures. Thune's office didn't respond to requests for comment, and a woman who picked up Kimberley Thune's cellphone hung up on a ProPublica reporter.

Thune also lobbied for the health system, then called Sioux Valley Hospitals, after an unsuccessful Senate bid in 2002, according to disclosure records. Thune, whose campaigns have received at least $8,000 in donations from Sanford, has lauded the billionaire from the Senate floor, calling him “a giving man with a passion for making a difference in the lives around him."

Sanford splits his time between homes in Sioux Falls, San Diego, Scottsdale, Arizona, and Vail, Colorado. In a 2007 Forbes profile that Sanford cooperated with, executives described him as a colorful boss, gathering his employees in his Phoenix winter home, cooking them breakfast in his underwear, and taking them out for bike rides and nights out dancing. His golf shoes are inscribed with his nickname, WOLT, which stands for World's Oldest Living Teenager.

Doris Burke contributed reporting.