Four more Mexican players have become embroiled in a doping probe, adding to the case of five further internationals found to have traces of clenbuterol at the CONCACAF Gold Cup which Mexico lifted last week, FIFA chief medical officer Jiri Dvorak said Friday. \"Five players tested positive for a relatively high concentration of clenbuterol (and) we have four others with traces of the same substance,\" Dvorak told Mexican broadcaster Televisa without revealing which players were concerned. Five Mexicans were suspended three weeks ago after they tested positive following results from a May 21 test during Mexico\'s pre-Gold Cup training camp. Dvorak said the cases were being followed up with further tests at a laboratory in Germany. Given the number of cases from the same team Dvorak said he was perplexed, saying it was \"difficult for me to imagine that practically a whole team from a developing country such as Mexico\" should turn in positive tests. He added it appeared that meat was contaminated with the substance while stressing that the substance is not a performance enhancer. After the original cases arose Mexican national team director Hector Inarittu brought the players to Los Angeles for further tests at the UCLA doping laboratory amid claims by the team that eating tainted beef and chicken caused the positive tests. Dvorak is in Mexico as part of a FIFA initiative to promote young people\'s health through playing football. The president of the Mexican Federation, Justino Compean, said that the results of an investigation into the clenbuterol cases could be ready as early as next week. The first five to test positive for the banned muscle-builder were goalkeeper Guillermo Ochoa, defenders Francisco Rodriguez and Edgar Duenas and midfielders Antonio \"Sinha\" Naelson and Christian Bermudez Ochoa insisted after the reports emerged of the initial five cases that the players were shocked. \"We don\'t have anything to hide. We are here to face the issue. We have a clear conscience because we haven\'t done anything that is incorrect,\" he said. The Mexicans believe eating tainted beef and chicken caused the positive tests. While US Anti-Doping Agency (USADA) reports have shown clenbuterol is used to promote growth in livestock, the drug is illegal in Europe and the United States. USADA calls the risk of tainted meat causing a positive result remote. Last week, FIFA president Sepp Blatter said he believed tainted meat was to blame - the same argument also used by Tour de France champion Alberto Contador after a positive test at last year\'s Tour.