Twenty-three players from Liga MX or featuring in matches involving Mexican Football Federation (FMF) teams tested positive for banned substances between 2012 and 2013, documents show.

Milenio Diario reported the findings this week after carrying out a freedom of information request with Mexico's National Institute of Transparency, Access to Information and Protection of Personal Data (INAI) in March 2014.

The FMF and Liga MX have never acknowledged or publicly released information that three out of 120 random drug tests in 2012 came back positive, as did 20 of 248 tests in 2013.

The tests from 2012 were only carried out on national team players (of all levels), while the tests from the second half of 2013 included Liga MX teams.

Liga MX president Enrique Bonilla denied that the positive tests -- carried out by Mexico's National Commission for Physical Culture and Sport laboratory, which is authorized by the World Anti-Doping Agency (WADA) -- broke any international or domestic rules, instead indicating that they were voluntary and the public didn't need to be informed.

"No institution obliges us to carry our anti-doping examinations," Bonilla told ESPN. "It is a voluntary decision. We do it every game."

Bonilla also revealed there are more cases since 2013 of players testing positive, but didn't give a figure.

In 2011, five Mexico national team players -- including Malaga goalkeeper Guillermo Ochoa -- tested positive for clenbuterol and missed the Gold Cup that year. However, the federation's disciplinary commission ruled the positive tests were due to contaminated meat and the players involved were not punished.

Bonilla stressed that the FMF is in contact with FIFA about the issue and implied there was no attempt to cover anything up.

"There's no way of hiding it," said Bonilla. "What's happened is that since the Gold Cup [in 2011] we've approached FIFA and the World Anti-Doping Agency to address the issue of clenbuterol."

At the 2011 Under-17 World Cup held in Mexico, FIFA found a majority of players tested had traces of clenbuterol in their system, although several athletes in other sports have been punished for its use and it remains a banned substance on the WADA list.

Mexico's Olympic Committee released information detailing the risks of clenbuterol in October 2015.

Mexican cattle ranchers are banned from using clenbuterol as a growth enhancer, but reports suggest that it is still used widely.

Two players in Liga MX tested positive for clenbuterol in August 2013, but their names were never released by the FMF, prompting the freedom of information request by Milenio.