Eight Nigerian artifacts that were probably stolen decades ago and illegally sent to the United States have been returned to the West African country by the Museum of Fine Arts, according to museum officials, who said Nigerian authorities planned to announce the transfer on Thursday.

The decision to return the artworks, including a 2,000-year-old terra-cotta head, was the culmination of an 18-month pursuit through dusty records and old gallery brochures, untangling an art-world mystery that spanned several continents. Along the way, the MFA discovered that one item, a brass altar figure, had probably been stolen from the royal palace in Benin City as recently as the 1970s.