World indoor 1,500-metre record holder Regina Jacobs, 40, has tested positive for the previously undetectable steroid tetrahydrogestrinone (THG), the U.S. Olympic Committee announced on Friday.

The committee also said that Sydney Olympics 4x400 metres relay gold medallist Calvin Harrison had tested positive for the stimulant modafinil.

Both positives came at June's U.S. championships in Stanford, California, and both athletes are disputing their cases, the USOC said.

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On Tuesday, U.S. women's hammer champion Melissa Price became the fourth American track and field athlete to test positive for THG.

Britain's European 100 meters champion Dwain Chambers has also tested positive for the steroid.

Price is married to British shot putter Carl Myerscough, who has served a two-year ban for failing a drugs test.

Discovery of THG has touched off an international doping scandal that has involved not only track and field, but U.S. professional baseball and football.

A federal jury is investigating the BALCO laboratory in San Francisco which the U.S. Anti-Doping Agency believes to be the source of THG.

Jacobs set the world indoor record of 3:59.98 last February and a month later won the 1,500-metre title at the World indoor championships in England.

She would face a two-year ban if her positive is upheld but her lawyer Edward Williams has denied the athlete knowingly took banned substances.

Harrison admitted in October he had testified positive for modafinil, saying he had been told by a coach who gave him it that it would keep him from getting too fatigued.

World 100 and 200 metre champion Kelli White also tested positive for the stimulant. Her case is under review and she could be stripped of her titles.

White has said she was prescribed modafinil to treat a sleep disorder.