The rare violin was on loan to Milwaukee Symphony Orchestra concertmaster Frank Almond. The robber used a stun gun on Almond and took the instrument from him Monday night in a parking lot behind Wisconsin Lutheran College, where Almond had just performed.
A retired FBI expert told the Journal Sentinel he expects the motive to be similar to that of high-end art thefts.
"Throughout my career, what I always saw in the end is that it was always about making money," said Robert K. Wittman, founder of the FBI's National Art Crime Team and author of "Priceless: How I Went Undercover to Rescue the World's Stolen Treasures."
"They are not about the pride of ownership or possessing an antique," he said. "The ultimate goal is either getting paid by someone to take it or trying to sell it in some type of market, or it could be a situation for them to get some type of payment for it to be returned."
Milwaukee Police Chief Edward Flynn has said the violin is valued in the "high seven figures." Investigators believe the instrument, known in musical circles as the "Lipinski" Stradivarius, was the primary target.
Wittman said thieves often are good at taking, but not at selling, an item of such value.
"They do a good job with the crime but never really think about what they're going to do with the item," he said. "They get stars in their eyes from the values, but the values for these types of things depend on authenticity, history and a good title - you have to own it, and they don't."
The robber fled to a nearby vehicle, described as a maroon or burgundy minivan driven by an accomplice. The violin case was found a short time after the robbery on the north side of Milwaukee, police said. The cases of such rare instruments usually contain a GPS tracker, according to experts, which could explain why the case was tossed.
Milwaukee police continued to investigate the crime Wednesday, along with the FBI's art crimes team based in Quantico, Va.