SAO PAULO (Reuters) - A deadly construction accident in November at the Sao Paulo stadium that will host the opening match of this year's soccer World Cup was probably caused by wet soil following heavy rains, newspaper Folha de S.Paulo reported on Friday. Citing an unpublished study by the Federal University of Rio de Janeiro, Folha said soil conditions were not solid enough to support a massive crane used to place the final section of roofing on the stadium. The crane fell, killing two construction workers on the ground. Workers were racing to complete the Arena Corinthians before a deadline, which was pushed back from December because of the accident, one of several at World Cup stadiums being built or refurbished around Brazil. The Sao Paulo stadium, owned by local soccer club Sport Club Corinthians Paulista, Is scheduled to host its first test match on May 17, less than a month before the World Cup kicks off. Another worker was killed at the stadium on Saturday after falling from the temporary stands being assembled to increase capacity for the tournament's prestigious opening match. Sao Paulo authorities have halted work on the stands pending a safety investigation, although work on the rest of the stadium has not been affected. According to the Folha report, the study concluded that a measure of soil firmness around the crane was only 13 percent, compared with optimum levels of nearly 80 percent. Officials at Corinthians and Odebrecht SA , the Brazilian company responsible for the construction of the stadium, did not immediately comment. Efforts to contact a spokesman for Liebherr, the company that owned the crane, were unsuccessful. According to Folha, Odebrecht has not had access to the study and declined to comment on its content. However, it reiterated that soil firmness was not an issue at the time, despite intense rainfall around November, and that the base structure for the crane was adequate. The 2014 World Cup, the first to be held in Brazil since 1950, has been beset by delays, repeated concerns about worker security and cost overruns. Besides the late delivery of stadiums, several important public transportation projects have been scaled back or abandoned altogether. The Arena Corinthians is among the most expensive of Brazil's 12 World Cup venues and was one of the last to get started. It is designed to seat 48,000 people, but an additional 20,000 seats are being added for use during the tournament. (Reporting by Guillermo Parra-Bernal; Editing by Todd Benson and Lisa Von Ahn)