The head of trustees at the British Museum said Saturday that the museum has recovered some of the 2,000 items believed to have been stolen by an insider, but admitted that the 264-year-old institution does not have records of everything in its vast collection.

Chairman of trustees George Osborne acknowledged that the museum's reputation has been damaged by its mishandling of the thefts, which has sparked the resignation of its director and raised questions about security and leadership.

Osborne told the BBC Saturday that 2,000 stolen items was a "very provisional figure" and staff were working to identify everything missing. The items include gold jewelry, gemstones and antiquities as much as 3,500 years old. None had been on public display recently.

He said the museum was working with the antiquarian community and art recovery experts to get the items back.

"We believe we've been the victim of thefts over a long period of time and, frankly, more could have been done to prevent them," he said. "But I promise you this: it is a mess that we are going to clear up."

Museum Director Hartwig Fischer announced his resignation on Friday, apologizing for failing to take seriously enough a warning from an art historian that artifacts from its collection were being sold on eBay. Deputy Director Jonathan Williams also said he would step aside while a review of the incident is conducted.

In early 2021, British-Danish art historian and dealer Ittai Gradel contacted the Museum bosses with his suspicions, but they assured him nothing was amiss. However, at the start of this year, the museum called in London's Metropolitan Police force.

The museum has fired a member of staff and launched legal action against them, but no arrests have been made.

Gradel told The Associated Press Friday he became suspicious after buying one of three objects a seller had listed on eBay. Gradel traced the two items he didn't buy to the museum. The object he bought wasn't listed in the museum's catalog, but he discovered it had belonged to a man who turned over his entire collection to the museum in 1814.

The historian said he found the identity of the seller through PayPal. He turned out to be the museum staff member who has since been fired.

Gradelsaid Williams had assured him that a thorough investigation found no improprieties. "He basically told me to sod off and mind my own business."

Fischer said in his resignation statement that "it is evident that the British Museum did not respond as comprehensively as it should have in response to the warnings in 2021." He also apologized to Gradel.

The thefts, and the museum's bungled response, have plunged the institution into crisis. 

The 18th-century museum in central London's Bloomsbury district is one of Britain's biggest tourist attractions, visited by 6 million people a year. They come to see a collection that ranges from Egyptian mummies and ancient Greek statues to Viking hoards, scrolls bearing 12th-century Chinese poetry and masks created by the Indigenous peoples of Canada.

The thefts have been seized on by those who want the museum to return items taken from around the world during the period of the British Empire, including friezes that once adorned the Parthenon in Athens and the Benin bronzes from west Africa.

"We want to tell the British Museum that they cannot anymore say that Greek (cultural) heritage is more protected in the British Museum," Despina Koutsoumba, head of the Association of Greek Archaeologists, told the BBC this week.

Osborne, a former U.K. treasury chief, said the museum has launched an independent review led by a lawyer and a senior police officer. He said it also had built a state-of-the-art off-site storage facility so the collection would no longer be housed in an "18th-century basement."

"I don't myself believe there was a sort of deliberate cover-up, although the review may find that to be the case," he said.

"But was there some potential groupthink in the museum at the time, at the very top of the museum, that just couldn't believe that an insider was stealing things, couldn't believe that one of the members of staff were doing this? Yes, that's very possible."