Saturday night's collapse in Indianapolis appears to be the deadliest of its kind in U.S. history, said Paul Wertheimer, founder of Crowd Management Strategies, a crowd-safety consulting company based in Los Angeles.
The Indiana State Fair stage toppled shortly after fair officials advised the crowd of roughly 10,000—gathered for a performance by the country band Sugarland—how to take shelter from an approaching storm.
Just after officials decided to order an evacuation—but before they could do so—a gust of wind ripped through the metal and fabric structure. The National Weather Service estimated that the gust exceeded 60 miles an hour. Along with the five people killed, 40 were sent to hospitals.
The fair reopened Monday with a memorial to those killed as county and state officials continued an accidental-death investigation.
"Everyone has questions about why this happened and how it happened," Sgt. David Bursten, a spokesman for the Indiana State Police, said Monday.
"It can take weeks to get those answers."
Outdoor stages like the one that collapsed are engineering feats: They must be portable and lightweight, yet support the weight of tens of thousands of pounds of equipment.
Industry standards for determining safe wind speeds are voluntary, and the building codes in many cities don't give much guidance on temporary structures of this sort.
"It is inconsistent across the country," said William B. Gorlin, vice president of the entertainment division of McLaren Engineering Group, which designed part of the stage for the current tour by Irish rock band U2. Mr. Gorlin said he wants state and municipal building inspectors to adopt his industry's standards on building outdoor stages.
Engineers say concert organizers can easily underestimate the danger created by a thunderstorm. Still, officials at engineering firms that design outdoor stages said Monday they were surprised by the collapse. A stage built to industry standards should have withstood such a gust, they said.
"The standards do exist," said Richard Nix, project coordinator at Entertainment Structures Group of Cincinnati, Ohio.
The outdoor stage in Indianapolis was built by Mid-America Sound Corp., of Greenfield, Ind. A spokeswoman said Monday that company executives were declining media interviews during the state investigation.
It was unclear Monday which government agency was responsible for inspecting the outdoor stage, and whether any inspection addressed the stage's ability to cope with storm winds.
"It's not us, and we don't know who it is," said John Erickson, a spokesman for the Indiana Department of Homeland Security, which oversees the state fire marshal and building commission.
Indianapolis officials said the city wasn't allowed to issue permits or conduct inspections on state-owned property such as the fairgrounds. The Indiana State Fair, a quasi-government state body, has closed the grandstands near the accident site.
The state police are working on the investigation with the Indiana attorney general's office, the Marion County coroner, the state fire marshal and the Indiana Occupational Health and Safety Administration.
Mr. Wertheimer, the crowd-safety consultant, said many bands and concert promoters are reluctant to cancel events due to weather because of bad publicity.
He applauded the decision by the Black Eyed Peas to cancel a free concert in New York's Central Park in June due to thunderstorms.
Indiana Gov. Mitch Daniels has defended the way state officials at the concert prepared the crowd for the threatening weather.
On Saturday, the same night as the stage collapse in Indiana, organizers cut short the Musikfest festival in Bethlehem, Pa., because of a lightning storm and flooding.
The weather has created mayhem at other outdoor concerts this summer. On Aug. 6, a Flaming Lips performance in Tulsa, Okla., was canceled after high winds caused overhead stage rigging to collapse. On July 17, wind caused a stage to collapse at Ottawa's Bluesfest while the band Cheap Trick was performing. Eight people were injured.
One of the worst outdoor stage accidents occurred Aug. 1, 2009, when stage scaffolding collapsed during a storm at the Big Valley Jamboree in Camrose, Alberta.
A woman was killed and 75 people were injured. Charges were filed in provincial court July 29 against festival organizers for failing to ensure worker safety and failing to ensure that equipment could withstand bad weather. The concert promoter said it planned to fight the charges.