Madonna's Sunday concert in Marseilles, France, has been canceled because of the deadly collapse of the stage that the U.S. pop star called a "tragedy."

The Times of London reported Friday witnesses to the Thursday accident at Velodrome stadium in Marseilles were being questioned by police as authorities attempt to discern the cause.

Technician Charles Prow, 23, and worker Charles Criscenzo, 53, died and up to 46 others suffered minor injuries and shock, including a U.S. national said to be in critical condition after the collapse. Two other victims were in serious condition, while the condition of the remaining victims was not reported.

"Two men lost their lives, which is a great tragedy to me. I feel so devastated to be in any way associated with anyone's suffering," said Madonna, who was scheduled to perform at the stadium Sunday.

The Times said the expected 40,000 attendees will be offered ticket refunds.