It was a “watershed” attack, “one in a million,” an all-but-unforeseeable “black swan.”

In the aftermath of the mass shooting at the Las Vegas country music festival, event security professionals — many with years of experience thwarting bad actors in bustling crowds — are characterizing the ambush in darkly exceptional, almost fatalistic terms. But they are also reckoning with ever-changing threats in their field after the aerial assaults that killed at least 59 people and injured more than 520 on Sunday.

The specter of calamity is especially worrisome for open-air events in urban environments — including the Austin City Limits music festival, which begins Friday in a Texas park and is now undergoing renewed security assessments.

“There is no manual for this,” said Chris Robinette, the president of Prevent Advisors, a security subsidiary of Oak View Partners, a company that advises sports and entertainment venues like Madison Square Garden. “It is a dynamic process that requires promoters, venue managers, local authorities and other stakeholders to work together.”

Ever since the Sept. 11 attacks, standard security protocol at concerts, festivals and other large entertainment events has become increasingly sophisticated, mirroring the mainstream adoption of previously unheard-of safety precautions at airports around the country. Music gatherings — long bastions of ephemeral intimacy and relaxed inhibitions — have become the site of bomb-sniffing dogs, body scanners and high-definition closed circuit cameras, particularly in the wake of recent large-scale attacks on concerts including the Bataclan rock club in Paris and the Manchester Arena.