(CNN) -- Three people have been arrested in connection to the case of a stolen Stradivarius in Milwaukee, but the violin remains missing.
Authorities are urging anyone with information to come forward.
"We do not have the violin. As is known, there is a substantial reward out for that violin, and we urge the community to do what it can to develop information and to help us identify, and locate, and recover this priceless instrument," Milwaukee Police Chief Edward Flynn told reporters Wednesday.
The reward is up to $100,000, according to CNN affiliate WITI, which reports the instrument itself is worth an estimated $6 million.
Flynn described the three suspects as a 36-year-old man, a 41-year-man, and a 32-year-old woman, all from Milwaukee. He did not identify them by name, and no one has been formally charged.
The suspects were arrested Monday and remain in custody.
"We're confident the subjects in custody are the subjects responsible for this," said G.B. Jones, acting special agent in charge with the FBI in Milwaukee.
At this point, Flynn said, authorities have no indication the three suspects were working for anyone but themselves.
He described them as "engaged in the process" and said that interviews are ongoing.
"Clearly, we want to indicate that it's perhaps in their best interests to recognize that ... this is not something that can easily be disposed of at some future date. It'll never be valuable for a thief. It's only valuable for a collector. It's only valuable to a collector if it can be played, and it can't be played if it's known to be stolen," the chief said.
The Stradivarius was stolen last week from Frank Almond, concertmaster of the Milwaukee Symphony Orchestra.
Almond was walking to his car after a performance on January 27 when an assailant used a stun gun on him, causing him to drop the instrument.
The attacker then took the violin and fled to a waiting vehicle, which was being driven by a second suspect, police said. That vehicle was described as a late 1980s or early 1990s, maroon or burgundy, Chrysler or Dodge minivan.
Authorities are still looking for that van.
According to the symphony, Almond plays on a violin made by Antonio Stradivari from 1715, known as the "ex-Lipinski."
Antonio Stradivari was born in 1644. He set up shop in Cremona, Italy, where he made violins, harps, guitars, violas and cellos.
Before his death in 1737, Stradivari is thought to have made more than 1,100 instruments -- of which some 650 remain today.
He is the world's most celebrated violin maker.