Scholarly expertise, a lengthy investigation and a new level of international cooperation recently resolved the theft of some irreplaceable Spanish and Italian medieval folios.

Samuel Banks, U.S. Customs Service deputy commissioner, returned four 13th- and 14th-century illuminated pages to Spanish Ambassador Antonio Oyarzabal late last month and the folios are on their way to Spanish cathedral libraries in Tortosa and Toledo. In June, the Customs Service returned three other folios to the Vatican Library in Rome.

During celebratory ceremonies at the Spanish chancery recently, J. Michael Marous, assistant U.S. attorney in Columbus, Ohio; Mark Beauchamp, a customs special agent; and Juan Hidalgo Cuesta, Spanish Embassy cultural attache, told the story to the Chronicler.

The mystery began on March 26, 1994, when Anthony J. Melnikas, a recently retired Ohio State University art history professor, consigned two 14th-century Spanish folios to Akron art dealer Bruce Ferrini.

Ferrini offered them in his catalogue as "King Freeing Slave" and "Dog and Man Fighting," named for the small and exquisite miniature pictures set in the text. The folios were priced from about $15,000 to $20,000.

A year later, Ferrini visited Melnikas's Upper Arlington, Ohio, residence to see what the professor called some important Italian folios; they were framed and hanging on his walls. Ferrini became suspicious and informed U.S. Customs Service officials. Photographs and photocopies were sent to James Marrow, a Princeton University art history professor.

Marrow took only a few hours of research to identify the pages as treatises on farming and war from the Vatican Library, commissioned and annotated by 14th-century poet-philosopher Francesco Petrarch. Father Leonard Boyle, the Vatican Library prefect, confirmed their loss and remembered that Melnikas had asked to see the manuscript in 1987.

Another art expert, Susan L'Engle,on leave from the Pierpont Morgan Library, traced two cutouts to the 13th-century "Gratiani Decretum" owned by the Tortosa Cathedral and two 14th-century papers to the Toledo cathedral library. One, the dog page, had been sold to a European collector, but payment was canceled after the investigation and the manuscript is expected to be returned. Marous estimated the value of the Spanish pages at $5,000 to $15,000 each.

Hidalgo, the cultural attache, learned during the investigation that since 1965, Melnikas had regularly researched medieval folios in the Toledo cathedral library, and perhaps as well in the Tortosa cathedral archives. In 1975 his scholarly treatise "Corpus of the Miniature in the Manuscripts of Decretum Gratiani" was published and favorably received.

"Gratiani Decretum," Hidalgo explained, means the legal scholar Gratian's canon and civil decrees with his commentary. "Back then there was no separation between the two," Hidalgo said. "The all-powerful church told everybody when to go to bed and when to get up." In the medieval centuries his scholastic milestone was widely copied.

Melnikas first told authorities he bought the items from a Spanish print-seller. Later, he offered other explanations for his acquisitions -- they came from a Roman poet, a Roman flea market, his wife's inheritance. He finally swore in a statement that, as far as he could remember, he found three loose manuscript leaves at the Vatican Library "and they ended up in my possession in the other papers -- research notes." (The Vatican pages have been valued at about $150,000.) But, he said, now that he noticed the edges had signs of being cut out, "it could be that I have done it."

"Melnikas visited the Vatican library hundreds of times," said Marous, who prosecuted the case. "He was considered one of the family or a piece of furniture around the library. He collaborated with the Vatican Library in publishing his 1975 three-volume work, and in 1987 was working with the library translating the Vatican's Gratiani Decretum' into modern Latin."

Marous added that Melnikas normally would do his research in the summer when the library was closed to the public, "often enabling him to be virtually alone and unsupervised in the manuscript reading room."

Putting all the pieces together took investigators 14 months. The case marked the first prosecution of trafficking in foreign-origin artifacts under the United States Archaeological Resources Protection Act.

A year ago this month, Melnikas pleaded guilty to eight federal charges of possessing and attempted sale of smuggled and stolen archaeological property.

The scholar, now serving a 14-month sentence, was fined $3,400 and ordered to pay $10,000 for the return delivery and restoration of the pages, as well as perform 250 hours of community service. CAPTION: Spanish Ambassador Antonio Oyarzabal displays one of the manuscripts recently recovered by the U.S. Customs Service. At left is Samuel Banks, U.S. deputy customs commissioner; at right, customs agent Mark Beauchamp. CAPTION: One of four recovered 13th- and 14th-century folios that are now on their way to Spanish cathedral libraries.