The following file, posted early today, has been updated to take in the latest developments and reaction from the swimmer, who proclaims her innocence:

Jessica Hardy did not test positive for a stimulant, strictly speaking, it is reported today. Rather, the substance in question is the anabolic agent - not a steroid, strictly speaking - clenbuterol.

The positive test was sandwiched between negatives and followed the 100m free at US Olympic trials. Hardy finished fourth and earned a berth on the 4x100m freestyle relay. The presence of clenbuterol is confirmed - even though the levels suggested at this stage appear to be barely beyond what is considered to be illegal - will make the breaststroke ace's chances of being cleared at appeal somewhat remote. A hearing will take place before the Olympic Games, it was confirmed today.

Here is precedence for use of the substance in swimming:

Xiong Guomin (CHN), tested positive for clenbuterol (a Beta-2 agonist) in an out-of-competition test on March 8, 1999, and was banned for two years in 1999. The Chinese Swimming Associated imposed a ban of one year on his coach, Xu Huiqin, who featured in the news broken by SwimNews this week.

Wei Wang (CHN) tested positive for clenbuterol (Beta-2 agonist) in an out-of-competition test on the same day as Xiong Guomin and was banned for two years. The Chinese Swimming Associated imposed a ban of one year on his coach, Cheng Zhi and extended the swimmer's ban to three years.

Ying Shan (CHN) tested positive for clenbuterol in a Chinese Swimming Assoication doping control on January 31, 2002.

Hardy faces a two-year ban in common with other cases involving the same substance. Hardy's attorney, Howard Jacobs, confirmed the drug positive. The swimmer's agent, Evan Morganstein, has backed his client, saying that she would 'never' cheat.

Hardy, a 21-year-old swimmer from Long Beach, qualified for the Beijing Games in the 100m breaststroke and the 50m freestyle, as wel as earning a berth in the 100m freestyle.

'Every day for the past four years I have had this in the back of my mind,' Hardy, a former water polo player who didn't start swimming until she was 16, said at the trials. 'I am so thankful and grateful that it has become reality.'

Her case is a blow to the clean image of US swimmers. 'Our athletes are like All-American kids,' said federation CEO, Chuck Wielgus recently. 'If you align yourself with them, you don't run the risk of athletes being found in some strip club in Vegas.' That remains the case. The manner of doping remains open.

Hardy is now at home with her family in Southern California while her case unfolds after she left the US training camp at Stanford. The US team leaves today, without her, for Singapore, where it will train until August 4 before moving into Beijing.

Wielgus said USA Swimming has been notified of the anti-doping case involving Hardy, whom he did not mention by name. 'The matter is being handled by USADA and we are hopeful that the matter will be resolved expeditiously,' he said in a statement.

Clenbuterol is one of five anabolic agents on the World Anti-Doping Agency's prohibited list. Although it has anabolic properties, it is not, strictly speaking, a steroid.

'It's a complex drug,' Dr. Don Catlin, the man who oversaw testing for anabolic agents at the 1996 Atlanta Olympics and who ran America's first anti-doping lab at UCLA for 25 years, told reporters this week. 'We know very little about it.'

Clenbuterol is readily available on the internet and is popular in the weight-loss market. Bizarrely, it is used legally in American horse racing because it can increase lung capacity, although it must clear a horse's system within a prescribed time before a race. Clenbuterol is also used to help asthma patients in some cases.

In September 2006, more than 300 people in Shanghai were poisoned by eating pork contaminated by Clenbuterol that had been fed to the animals to keep their meat lean. 'It can be pretty toxic,' Catlin said. 'There have been some epidemics where human beings have ingested it by ingesting meat and that has given them some pretty bad reactions. That's surely one of the reasons it doesn't get into the US'.

In Hardy's case, somehow it got into her sample and she will, under the current testing system, now have to give a cast-iron explanation as to how the substance came to be in her blood stream. If guilty, no explanation is needed; if innocent, explanations are possibly impossible to find. We await more details about how the substance may have ended up in the testing bottle.

Hardy spoke on CBS TV's The Early Show, saying: 'It's heartbreaking and devastating.It's literally a nightmare. In my heart I know I'm 100 per cent clean and I've never done anything different my whole career. I've been clean my whole career and to have this huge setback ... it's just really heartbreaking.'

Hardy said she was bewildered by the test result. 'I have my attorneys and my experts looking into it, but honestly we have no idea how this positive test happened.'

She recalled the moment when she learned of the problem: 'USADA, the US Anti-Doping Agency, called and said that I tested positive and I had never even heard of this drug before. I was taking notes right when she called, to write down my information and everything and I spelled the drug name wrong even, and complete shock. I was devastated. Called my parents, had my parents call an attorney. Just took the next steps. But most of it was just dealing with the shock and really just having emotions.'

She added: 'We're going to have a hearing before the start of the Olympic Games, and as soon as possible to try to make sure that I can compete, because I know that I'm innocent and we just have to prove this.'

Meanwhile, Tara Kirk, third at trials in the 100m breaststroke, believes that someone has let her and Lara Jackson, third in the 50m free, down badly by failing to notice that the July 21 Olympic entry date was looming and that getting the news of any positive test out in the public domain was imperative to getting replacements on to the US team in Beijing. Her personal blog tells the tale of how she feels.

And how Brazil's Rebeca Gusmao feels is also pretty much down in the mouth: she has been served another two-year ban for elevated levels of testosterone in a test taken in 2006, the year before the test that caused her downfall in the first instance, at the 2007 Pan American Games in Rio de Janeiro, led to her suspension. Gusmao is in the process of appealing.