The probe found that some of the works had missing or forged export documents, while others had vanished decades ago from West African collections before showing up mysteriously on the U.S. art market.

"These objects have gaps where there shouldn't be gaps," said Victoria Reed, the museum's curator of provenance. She said the works had been sent back to Nigeria earlier this month.

The pieces were part of a broader collection bequeathed to the museum last year by the late William and Bertha Teel, of Marblehead, Massachusetts, a coastal town about 18 miles north of Boston.

Museum officials said the Teels were not aware of the artifacts' dubious origins and had acquired them in good faith from dealers.

Some of the objects raised immediate concerns, including three terracotta pieces from Nigeria's Nok culture, a type of work listed by the International Council of Museums as being at high risk of having been stolen.

Another piece, a bronze figure dating to about 1914, was known to have been in Benin City's Royal Palace in the 1970s. But that palace was robbed in 1976, and the figure's whereabouts were next placed in a gallery in the 1990s.

Another work known to have been in Nigeria's Oron museum in the 1970s went on to mysteriously appear on the art market decades later, said Reed.

Last year, Reed wrote to Nigeria's National Commission for Museums and Monuments to ask if the commission had approved the export of the eight flagged pieces. The commission replied that it had not done so, and the lone export document for one of the pieces was found to have been forged, said Reed.

The Museum of Fine Arts, which has a collection of about 500,000 objects, has returned a total of 27 objects to their home countries since the 1990s, not including the eight Nigerian works, according to the museum. Reuters