We don’t know the reason yet.
A heavy security presence is reported by audience members.
Refunds are being offered against future performances of the same opera.
We hear from an orchestra player that a man was observed sprinkling a white flaky powder substance from a bag on the timpani, and then the podium. It was reported to security and house management. The powder is being tested. Peter Gelb cancelled the show according to security protocol.
There appears to be no ongoing threat.