The billionaire owners of anti-abortion craft giant Hobby Lobby are under federal investigation for allegedly looting hundreds of ancient artifacts from the Middle East for use in their personal “Museum of the Bible." The family has been under investigation since 2011 after Memphis customs seized nearly 300 clay tablets headed for the Hobby Lobby headquarters in Oklahoma City.

The Green family has an estimated 40,000 artifacts for their Museum of the Bible, a nonprofit scheduled to open in 2017 in Washington, D.C. The tablets seized in Memphis are inscribed with cuneiform script, which dates back thousands of years to Iraq.

The seizure of the cuneiform tablets and the federal investigation were confirmed by Cary Summers, president of the Museum of the Bible, as well as members of the Green family, the Daily Beast reported exclusively Monday. “Is it possible we have some [illicit] artifacts? That’s possible,” CEO Steven Green told the Daily Beast.

Summer said the looting was unintentional. “There was a shipment and it had improper paperwork—incomplete paperwork that was attached to it," he told the Daily Beast.

The Green family will have to pay a fine if they are prosecuted in the artifact case. The Greens, who are worth $4.5 billion, became famous after they challenged a federal healthcare mandate to help cover employees' contraception. The Supreme Court ruled last year that they did not need to supply female employees with contraception.

“Protecting the free-exercise rights of corporations like Hobby Lobby, Conestoga … protects the religious liberty of the humans who own and control those companies,” Justice Samuel Alito wrote at the time.

The FBI has warned art collectors and dealers about Near Eastern antiquity trading in recent months as Islamic State group militants have begun selling on the marketplace. “We now have credible reports that U.S. persons have been offered cultural property that appears to have been removed from Syria and Iraq recently,” Bonnie Magness-Gardiner, manager of the FBI’s Art Theft Program, said in a press release in August.