A performance of Guillaume Tell was stopped after an audience member sprinkled a powder into the orchestra pit.
The incident, which happened at the Metropolitan Opera in New York, prompted anti-terror units to storm the theatre.
The New York Police Department was notified and said that the person who sprinkled the powder fled the scene and is being sought.
Although no one knows yet what the mysterious substance was, there were no reported injuries.
The show was cancelled, and the venue also cancelled a later performance of L’Italiana in Algeri while authorities investigated.