An original copy of a 12th-century illuminated manuscript of incalculable historical and cultural value has disappeared from the cathedral of this northwestern Spanish city.

The Codex Calixtinus was reported missing Tuesday from a heavily guarded, armored vault in the cathedral's archive, staff said.

A police spokesman told Efe that investigators were analyzing the security cameras for clues to the disappearance of the manuscript, which contained homilies and practical advice for medieval pilgrims following the Way of St. James.

The missing codex is an original work and not the replica that is enclosed in a glass case inside the cathedral and displayed to tourists.

The original text was only brought out of its safe on special occasions, the last time when Spanish Culture Ministry personnel paid a visit to the cathedral.

The codex, which is of great artistic and historical importance, is believed to have been commissioned by Pope Callixtus II (hence the name) to promote pilgrimages to the shrine in the 12th century.

A large law-enforcement operation has been launched to find the missing codex, involving police and forensic experts in Santiago de Compostela and experts from a police unit in Madrid specializing in crimes against the country's cultural heritage.

The cathedral's dean and senior archivist, Jose Maria Diaz, said there was no sign of forced entry into the vault containing the codex and that only he and two other people had access to the room.

He added that the vault contained several other highly valuable documents owned by the cathedral but the codex was the only item missing.

"Whoever took it knew its importance, knew its incalculable value and knew how to get to it," he said.

The biggest danger to the manuscript is that the color of its illustrations could be altered and its pages may be damaged if exposed to improper light and humidity, the director of the University of Salamanca's General Historical Library, Margarita Becedas, said.

The Codex Calixtinus is composed of five volumes with a variety of themes but its singular objective was to extol the virtues of St. James the Apostle, whose remains were said to have been found in Santiago de Compostela in an urn some eight centuries after his death.

The missing illuminated manuscript is considered the oldest and most complete original copy of the Codex Calixtinus; 200 others are distributed across Europe in the Spanish cities of Barcelona, Madrid and Salamanca, as well as in Lisbon; London; Pistoia, Italy; and the Vatican.

The most well-known and most frequently translated of the five volumes is the last, which served as a guide for the medieval Way of St. James pilgrim and describes the route, its towns and cities, its people and customs and shrines that should be visited.

European protocols to monitor markets in which such a singular work might be sold have been activated, Miguel Cortizo, the delegate of the regional government of Galicia, where Santiago de Compostela is located, said.