Three rhinoceros horns worth tens of thousands of pounds have been stolen from a museum's secure storage area.
The items, which have been in New Walk Museum's collection for 50 years, may have been stolen for sale on the black market.
Rhino horns and elephant tusks can fetch tens of thousands of pounds in some Asian countries, where they are used as ingredients for traditional medicines and, in some cases, aphrodisiacs.
Police were informed of the theft in September, when staff were sure the horns had been stolen and not simply mislaid.
It is unclear when the horns were taken, but the theft is believed to have happened in the past few months.
Leicester City Council, which runs the museum, said it had improved security since the theft came to light.
The items have not been on public display for many years because of the authority's opposition to the trade in endangered species' body parts.
A 57-year man was arrested and has been released on police bail while officers continue their investigation.
Last year, European police agency Europol warned that crime gangs were targeting museums, particularly those in smaller towns, specifically for rhino horns.
Dr Dave Unwin, of the School of Museum Studies at the University of Leicester, said the horns could be sold for tens of thousands of pounds each on the black market in Asia.
Dr Unwin said: "Rhino horns and, indeed, elephant tusks, are extremely valuable in Asia, where they are ground up and sold as medicine or aphrodisiacs.
"There has been a spate of these thefts across the country in recent years and these items can be sold for tens of thousands of pounds.
"So, the New Walk Museum is certainly not the first victim, but what is unusual here is that these items appear to have been stolen from an area which wasn't easily accessible to the public."
A report by Europol last year highlighted the theft of rhino horns as a growing problem in the UK.
The report said: "The theft of rhino horn shows how organised criminals are always on the look-out for new and creative crime opportunities.
"The attraction towards rhino horn clearly stems from the high value of this rare commodity.
"Depending on the size and quality of the specimen, a rhino horn can be worth between 25,000 and 200,000 euros.
"Rhino horns are used for decoration, to produce luxury products and also used in traditional medicine, despite it being proven ineffective."
The report highlighted one Irish gang's methods.
It said: "To source and acquire rhino horns, the group has targeted antique dealers, auction houses, art galleries, museums, private collections and zoos, resorting to theft and aggravated burglary where necessary.
"To sell specimens, they have exploited international auction houses in the UK, France, USA and China."
A city council spokesman said: "We can confirm that three rhino horns went missing from the museum stores in September.
"The practice of hunting rhinos for their horns has decimated the population of several species and the horns would never have been exhibited.
"But they have been in our possession for more than 50 years and we were very disappointed that they had been stolen.
"Since discovering the theft we have had a complete review of security and access to our stores, and implemented new and more stringent procedures."
Leicestershire police confirmed it was investigating the theft.
