Four Russian track-and-field athletes have tested positive for the banned drug meldonium, Russia’s athletics chief said on Monday, a disclosure which could further damage Moscow’s efforts to overturn a doping ban in time for the Rio Olympics.

At least 16 Russian sportsmen and women, including world tennis star Maria Sharapova and speed skating Olympic gold-medallist Semion Elistratov, have been caught using meldonium since it was banned by the World Anti-Doping Agency on Jan. 1.

Despite warnings from sports officials that a number of other Russian competitors could have taken the substance, Sports Minister Vitaly Mutko told Reuters on March 11 that meldonium had nothing to do with athletics in his country.

But the Russian Athletics Federation (ARAF) said on Monday four athletes had tested positive at the Russian Winter Indoor Athletics Championships in February.

“We have information that four people gave positive tests for meldonium. We will deal with this today,” the Interfax news agency quoted ARAF head Dmitry Shlyakhtin as saying.

One of the athletes named was Russian long-distance running champion Andrei Minzhulin. It was unclear whether sprinter Nadezhda Kotlyarova, who revealed on Sunday she had taken meldonium, was included among the four.

The scandal will complicate Russia’s campaign to prove it is compliant with anti-doping standards after being suspended from international competition last year following revelations of widespread cheating and corruption.

If it cannot get the suspension lifted, Russian athletes will miss the Olympics starting in Rio de Janeiro on Aug. 5 – a humiliating blow to a country that has long drawn pride and prestige from its record as an athletics superpower.

The International Association of Athletics Federations (IAAF) did not immediately reply to a request for comment.

Minzhulin, who won the men’s 5,000-meter final at the Russian championships in February, said 55 nanograms of meldonium had been found in his blood. He and Kotlyarova both said they stopped using the drug before it was banned.

“I admit the fact of taking meldonium, but I took the substance when it was not considered doping and stopped taking it in good time,” R-Sport news agency quoted him as saying

Meldonium has a half-life of between four and six hours but can remain in the human body for significantly longer, said Latvian manufacturer Grindeks.

“Terminal elimination from the body may last for several months,” said Grindeks spokeswoman Ilze Gailite in written comments to Reuters.

“It depends on a variety of factors such as dose, duration of treatment … type of samples (blood or urine) used for detection of the substance.”

Since former world tennis number one Sharapova admitted using meldonium, at least 100 athletes from multiple countries have tested positive for the drug, which is used to treat diabetes and low magnesium and has been linked to increased sporting performance.

Meldonium is particularly popular in Russia and the former Soviet Union, however, having been invented in Latvia and used to help Soviet soldiers fight at high altitude in the 1980s.

R-Sport reported that around 40 Russian athletes from more than 10 different sports had tested positive for meldonium in the first two months of 2016.

Mutko said on Monday the cases of meldonium used in Russian athletics should not be connected to his country’s preparations for the Olympics, and athletes would be tested at least three times before leaving for Rio.

“Everyone who is preparing for the games is being monitored. But the issue of meldonium is a separate issue,” TASS news agency quoted him as saying.

“The testing plans are completely clear. Our sportsmen have been warned – they can be tested at any time. Their task is to prepare for international competition.”