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Maria Sharapova at the 2014 U.S. Open. (AP Photo)

MOSCOW -- The obscure performance-enhancing drug that may have doomed tennis star Maria Sharapova's career was developed for the Soviet military and used during the Soviet invasion of Afghanistan to maintain soldiers' focus under stress, according to the drug's inventor.

Sharapova announced Monday that she tested positive at the Australian Open for meldonium, which has been banned in competition since Jan. 1. The drug, which is not approved for use in the United States, is used to treat heart disease and other chronic conditions in Russia and Eastern Europe -- but it was first developed in the then Soviet republic of Latvia at the height of the Cold War as a military tool.

What a long journey from the battle-scarred valleys of Afghanistan to the manicured courts of Wimbledon.

Meldonium, also known as mildronate, "was invented in the Soviet era, when Ivars Kalviņš was studying mechanisms of stress on the body," according to a 2009 article in the weekend supplement of Latvia's Diena newspaper.

"His research results coincided with the authorities' demand to create a medicine that enhances endurance and performance. So the medicine, first produced in small quantities in laboratories, became a part of a nearly everyday diet for [Soviet] soldiers – it was used by pilots and those who served on submarines, as well by those who took part in the [Soviet] war in Afghanistan," Diena wrote.

“High altitude. Oxygen deprivation. If you have to run 20 kilometers with your full kit, you wind up with ischemia," Kalviņš told Diena in 2009, referring to a shortage of oxygen in the heart muscles. "All of them were given mildronate. They themselves didn’t know what they were using. There no one asked any questions."

The drug had "six or seven different uses" by the 1991 breakup of the Soviet Union, the newspaper wrote.

Meldonium is still made in Latvia, a point of pride for the tiny Baltic nation of 2 million people. It is well-known in the former Eastern bloc despite being unavailable in the West.

Sharapova said that she had not been aware that the drug had been banned, and that she had taken it for a decade because of a family history of diabetes and low magnesium levels.

Kalviņš has said repeatedly that he does not believe taking the drug creates an unfair athletic advantage, and he has said that he and Grindeks, the Latvian company that manufactures the drug, intend to appeal the decision to place it on the banned list.

Sharapova became just the latest Russian athlete to become engulfed in a doping scandal. Last fall, the Russian Anti-Doping Agency was accused of colluding with athletes to warn them about tests in advance, and the doping appears to have been so systematic that all Russian athletes are currently banned indefinitely from international competition.

Russian Sports Minister Vitaly Mutko said Tuesday that he expected more Russian athletes to fail tests for meldonium, and that the country was still working to rebuild its anti-doping system after the November scandal.

"A lot of athletes took this drug," Mutko told the Tass news agency on Tuesday, adding that he did not believe it created any unfair advantages.

Kristine Berzina contributed to this report.