CHICAGO (AP) _ A vintage viola that was lost almost two years ago is back making music with the Chicago Symphony Orchestra thanks to a quick-witted violin shop owner.

The orchestra's principal violinist, Charles Pikler, left the instrument behind on a downtown Chicago sidewalk in September 1996. Within minutes, it vanished.

``I was horrified,'' Pikler said Sunday. ``I was frantic. I went to the first phone and started calling the police.''

Crafted in 1723, the viola's inscription identifies the maker as Dominicus Montagnana, one of the greatest Venetian violin makers of his time. But the instrument's authenticity is disputed.

Its value is estimated at $30,000 to $1 million, depending on its authenticity.

Orchestra officials thought they had lost the instrument for ever, but it turned up last week in the hands of a 71-year-old Chicago man who tried to sell it to a violin expert.

Robert Heiss was charged Friday with one count of felony theft. He claims he bought the instrument at a flea market for $20,000.

``We kind of wish that it could talk so we would know where it's been,'' CSO Vice President Vanessa Moss said Sunday.

The viola was turned over to police by Fritz Reuter, a violin expert who runs a music shop in Lincolnwood. Reuter said he became suspicious when Heiss was vague about how he had acquired the instrument.

Pikler was called in to identify the viola, which he did by giving an impromptu performance for several bemused officers to make sure it sounded right.

On Saturday, Reuter received a large bouquet of flowers from the orchestra with a card that read, ``With unimaginable gratitude from your friends at the Chicago Symphony Orchestra.''