Masked men stormed a warehouse and stole stuffed rhinoceros heads with eight valuable horns from Ireland's National Museum. Police and museum officials said Thursday that the heist was probably the work of an Irish Gypsy gang known for stealing rhino horns.

The museum's storeroom in Swords, north of Dublin, was raided on Wednesday night, according to police. A single security guard was tied up, but he later got away and notified authorities.

The haul of horns, each around 100 years old, is worth a total of about (EURO)500,000 ($650,000) on the black market based on their weight.

Rhino horns command high prices on the black market because three of the five species of rhinoceros in Africa and South Asia have been hunted to the verge of extinction. The powdered horns are used in traditional Asian medicine, primarily in China and Vietnam,

The Irish Gypsy criminal network that is likely behind the theft is known officially as the Rathkeale Rovers. They have been behind a number of rhino horn thefts internationally.