FIFA released a letter from the Qatar organizing committee which said it regarded The Guardian newspaper's investigation with "utmost seriousness."
Blatter's courtesy visit to Qatar was dismissed as "totally inadequate" by the Brussels-based International Trade Union Confederation, which first met with FIFA on the issue in 2011.
"Scores of healthy young men are dying. This web of deadly practices draws in international companies, the government of Qatar and FIFA," Sharan Burrow, the ITUC general secretary, said in a statement.
The FIFA executive committee included 13 men who took part in the 2010 vote. Qatar beat the United States 14-8 in the final vote despite warnings that the extreme temperatures in June and July posed a health risk to players and spectators.
Qatar has twice hosted major football tournaments but neither was played in June or July, nor even November. The 2011 Asian Cup was played in January, and the Under-20 World Cup in April 1995.
The U.S. is FIFA's most lucrative territory, and November or January dates would clash with the NFL season. Fox, Telemundo — owned by NBC Universal — and Futbol de Primera Radio agreed to pay a combined $1.2 billion in October 2011 to broadcast the 2018 and 2022 World Cups.
FIFA has ruled out compensating losing bidders after 2022 candidate Australia said it wanted to recoup its publicly funded $40.6 million campaign costs.
FIFA gets about 90 percent of its revenue from the World Cup. It earned $3.655 billion from commercial deals tied to the 2010 tournament in South Africa.
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