Crews on Sunday sifted through debris of a concert stage that collapsed at the Indiana State Fair, trying to determine what caused the accident that killed five people and injured 40, authorities said.

The fairgrounds were closed Sunday as investigators began the search for answers. The state fair was expected to reopen to the public Monday following a public memorial service to remember those who were killed and injured, said Cindy Hoyne, the fairs executive director.

Forecasters warned heavy rain and strong winds would hit the fair nearly two hours before the storm moved through, causing the collapse of the stage Saturday night. The National Weather Service estimated winds at 60 to 70 mph.

Metal scaffolding fell onto the Sugarpit, a section usually occupied by Sugarlands most ardent fans, about four minutes after authorities took the stage to warn the crowd to seek shelter, according to a timeline of events released by investigators Sunday.

Its not clear to me at this stage how anyone could have foreseen a sudden, highly localized gust of wind in one place, Gov. Mitch Daniels said.

Video shows the blue canvas top fraying and flapping just seconds before the steel scaffolding gave way, sending a heavy bank of stage lights and metal onto fans closest to the outdoor stage.

Gov. Daniels described the collapse as a freakish accident.

Sugarland, an Atlanta based country duo, escaped injury since they had not yet come onstage. They later posted a message on Twitter saying they were stunned and heartbroken for the fans.

The group along with its opening act, Sara Bareilles cancelled their scheduled performance Sunday night at the Iowa State Fair. According to the fairs website, aside from coping with the magnitude of the accident, the ongoing investigation prevents the band from getting what may remain of their stage equipment.

Jennifer Nettles, one half of the Sugarland duo, posted a statement on the groups web site Sunday, saying there are no words to process a moment of this magnitude and gravity.

There are only prayers for peace in the hearts of the bereft, and prayers of thanksgiving for those who were spared or safe. A piece of our heart is left in that grandstand, Nettles said.

Sara Bareilles, who had just finished her performance when the collapse occurred, tweeted Sunday she was still thinking of all those affected.