A gang of thieves trying to steal rhinoceros horns from museums across Europe has struck again, but this time they came up empty.

British police say the thieves broke into the Natural History Museum at Tring, northwest of London, early Saturday and ripped off the horns from two stuffed rhinos that were on display. Museum officials say the horns were fakes, put in after a series of thefts at other museums.

The museum remained closed Saturday while investigators looked for evidence that could lead them to the thieves.

Museum manager Paul Kitching said the museum was deeply saddened by the break-in and called the theft pointless. But this latest incident is worrying to police, who think the illegal trade in rhino horns is expanding.

Similar rhino horn thefts have been reported It Italy, Germany and Britain. And experts say the horns – used as aphrodisiacs or medicine in Asia – have become more valuable than diamonds, gold, heroin or cocaine.

Police say the rare horns are also in demand among black market collectors.

Museums across Europe have been warned to remove any rhino horns from display or replace them with replicas made of resin or other material.

While the horns taken from the Natural History Museum at Tring were replicas that have no value, the real horns would have been worth almost $400,000.

The Irish Times reports Europol believes an Irish gang may be to blame for many of the heists.