CHICAGO (AFP) - A "priceless" Stradivarius violin remained missing on Wednesday after United States police nabbed three thieves who attacked the concert master of the Milwaukee Symphony Orchestra as he was leaving a performance last week.
Police said they believe the armed robbery was a "theft of opportunity" and that the thieves were not part of a crime ring.
"At this point we don't have any indication that they were working for anyone but themselves," Milwaukee police chief Edward Flynn told reporters.
Around 600 violins made by Italian master craftsman Antonio Stradivari are still in existence and they are prized for their incredible - and inimitable - sound.
One fetched about 11 million euros (S$18.9 million) in a 2011 charity auction for victims of the Japanese tsunami.
Named after an early owner, the Lipinski Stradivarius stolen last week was on loan from an anonymous donor whose Wisconsin-based family had owned it for more than 50 years.
The armed thieves set upon concertmaster Frank Almond after an evening performance at Wisconsin Lutheran College at 10.20pm on Jan 27 (12.20pm on Jan 28, Singapore time).
Police were able to find the discarded case within a matter of hours but have so far not had any luck convincing the thieves - who were arrested on Monday - to tell them where to find the violin itself.
Mr Flynn said the department is working to convince the thieves that the violin is not a "trophy" and cannot be easily sold.
"It can only be valuable to a collector, and it can only be valuable if it's played," he told reporters.
He does, however, believe that the violin is "still in our jurisdiction".
"We urge the community to help us identify, locate and recover this priceless instrument," Flynn said noting that a $100,000 reward has been offered for information that leads to the violin's safe return.
Police did not release the names of the thieves, who are all from the Milwaukee area and were identified only as two men, aged 41 and 36, and a 32-year-old woman.
The Federal Bureau of Investigation's art theft division is assisting in the investigation.