CHUNCHEON (Yonhap News) -- The national marathon team head coach is being probed for allegedly doping his athletes, including an Asian Games gold medalist, local police said Friday.

Jeong Man-hwa has allegedly made his marathoners inject themselves with illegal substances that increase red blood cells since April, said a spokesman for the police here, about 85 kilometers east of Seoul, adding these substances are undetectable in drug tests.

Police are also examining records at a rehabilitation hospital in Jecheon, North Chungcheong Province, where most of Jeong's athletes were treated and may have been injected with banned substances.

Among the athletes is Ji Young-jun, a top local marathoner who won the gold medal at the Guangzhou Asian Games last November.

Police said they've already questioned two athletes and will expand their probe into about 20 others, including Ji.

Substances that control red blood cell production are known to improve stamina for athletes. Cyclists, distance runners, triathletes and cross-country skiers have been known to use erythropoietin (EPO), a high-profile blood doping substance.

The investigation comes about two months before the southeastern provincial capital city of Daegu hosts the 2011 World Championships in Athletics. Korea has never won a medal at the world championships, and the marathon is regarded as the host's best chance for a medal.

"We're being careful with this investigation because it could send huge shock waves throughout the athletics community ahead of the world championships," a police investigator said. "We will announce the results of our probe as soon as our analysis is done."

According to the Korea Association of Athletic Federations (KAAF), Jeong has told KAAF President Oh Dong-jin that he is innocent, and Oh urged Jeong and the marathon national team to stay focused on their preparations for the world championships.

KAAF officials said their own investigation found no evidence of doping among marathoners, insisting someone conspired against Jeong and gave false tip-offs to police.

Jeong had been a coach for a girls' middle school and high school in Gangwon Province. KAAF officials said some coaches of semi-pro clubs became angry with Jeong when he refused to let them recruit his athletes.

"President Oh said after the world championships, he will try to identify just who made these false accusations and take legal action against them," said Seo Sang-taek, the KAAF director of planning and public relations.

"No athlete has tested positive in recent competitions," Seo added. "The Korea Anti-Doping Agency (KADA) told us that these blood cell-making agencies can be detected in tests."

Hwang Kyu-hoon, a vice president of the KAAF and former coach of distance runners, said allegations against Jeong "aren't even worthy of our time."

"Today, event organizers run strict tests on athletes, including urine and blood tests," Hwang said. "So if you've doped, you're bound to get caught sooner or later."