It should have been one of those nights musicians live for. Frank Almond, the concertmaster of the Milwaukee Symphony Orchestra for nearly two decades, had just closed a chamber concert in his own “Frankly Music” series with Messiaen’s hushed, eerily intense “Quartet for the End of Time.” Mr. Almond drew the graceful, ringing high notes of the finale from his prized 1715 Stradivarius violin, producing a tone so intensely focused that the audience in the Wisconsin Lutheran College’s 388-seat auditorium sat in awed silence for 20 seconds before applauding.

But the glow of the moment evaporated quickly, once Mr. Almond, 49, stepped into the college art center’s parking lot at 10:20 p.m. Monday, his violin carefully swaddled against the subzero temperatures and minus-25-degree wind chill. And as he neared his car, a figure stepped up to him and shot him with a stun gun.

It happened in a matter of seconds: Mr. Almond dropped the violin, the attacker scooped it up and jumped into a late 1980s or early ’90s maroon or burgundy minivan, where an accomplice was waiting to speed away. Edward A. Flynn, the Milwaukee police chief, said late Thursday afternoon that Mr. Almond had described the thieves as a man and a woman. Chief Flynn has given the value of the violin as “the high seven figures.” The police said earlier that the violin’s empty case had been found several miles from the hall.

A spokeswoman for the orchestra confirmed that the instrument was insured, but said that because of the investigation, she could not provide details about the amount, or what restrictions, if any, applied to the use of the instrument. Given its prominence — high-resolution photographs of Strads are plentiful — it would be virtually impossible to sell the instrument on the open market.