Marion Jones' latest comeback came in a laboratory, where her backup sample turned up clean — a stunning twist that clears her to compete and could validate a long list of triumphs sullied by years of doping allegations.

The "B" sample taken from one of the world's best-known and most decorated sprinters did not detect the banned endurance enhancer EPO, her attorneys said Wednesday night.

The finding means her initial positive result is thrown out, clearing her of the most recent — and most damaging — allegations against Jones and paving the way for her return to the track.

"I am absolutely ecstatic," Jones said in a statement released by her lawyers. "I have always maintained that I have never ever taken performance enhancing drugs, and I am pleased that a scientific process has now demonstrated that fact."

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Long a target of governing bodies in track and the Olympic movement, Jones always vehemently denied ever taking performance-enhancing drugs. But she tested positive for EPO on June 23, after winning the 100 meters at U.S. nationals for her first sprint title since 2002.

She withdrew from the 200 meters the next day and was slated to race at a meet in Switzerland in August, but withdrew unexpectedly. Hours later, reports of her positive "A" test for EPO were revealed.

She faced a minimum two-year ban, pending the result of the backup, or `B,' test, conducted at the same UCLA lab using the same sample.

"I am anxious to get back on the track," the 30-year-old sprinter said.

The International Association of Athletics Federations is awaiting notification of the lab result.

"We have not received any formal notification from the lab," IAAF spokesman Nick Davies said Thursday. "Until then, we can't comment. Only until we receive the notification and accept the explanation can we make any decision on her eligibility."

The statement about the results, released by attorney Rich Nichols, said Jones was informed of the negative test by the U.S. Anti-Doping Agency. USADA does not comment on active cases and never acknowledged Jones' positive `A' test.

USADA general counsel Travis Tygart did not immediately return messages left late Wednesday by The Associated Press. U.S. Olympic Committee spokesman Darryl Seibel said the federation had no comment.

Questions have long been raised about the reliability of EPO testing, and this negative 'B' test will spark further debate.

"I believe there are issues with that test," said Howard Jacobs, another Jones attorney who has defended several athletes on doping charges. "It's a difficult test. From what I saw on the 'A' sample, it was questionable as to whether it should've been called a positive. I can't say I was shocked that the 'B' came back negative based on what the 'A' looked like."

As he has in the doping case involving Tour de France winner Floyd Landis, Jacobs derided the leaking of positive tests. Doping cases aren't supposed to be made public until they are resolved, but most are reported once a positive 'A' test is confirmed.

"This is perfect illustration of why this new trend of leaking A-positives is a horrible thing," Jacobs said. "This whole thing should have happened anonymously. Marion should've been able to keep competing and no one should have known about it."

EPO is also known as Erythropoietin, a banned performance-enhancer that can boost endurance.

Jones, who has five world championships to go with her Olympic medals, dominated track and field in the late 1990s. At the Sydney Games, she became the first woman to win five Olympic medals — taking gold in the 100 meters, 200 meters, 1,600-meter relay and bronze in the long jump and 400-meter relay.

Since then, her reputation has suffered. She is one of several athletes who has testified to the federal grand jury investigating BALCO in 2003. Her ex-husband, C.J. Hunter, and Bay Area Laboratory Co-operative founder Victor Conte have accused her of using banned substances, allegations she has denied.

Her former coach, Trevor Graham, has been linked to several athletes in trouble for doping, including Justin Gatlin, who recently tested positive for testosterone or other steroids and faces a possible eight-year ban. Last December, the father of Jones' son, sprinter Tim Montgomery, retired after he was banned for two years for doping violations — the result of information gathered in the BALCO probe.

Earlier this year, the International Olympic Committee said it would continue to investigate Jones' performance in Sydney to determine whether she was doping then.

Jacobs said the turnaround in Jones' case places a burden on sports federations and those who administer the tests to make sure they're doing a good job and following protocol on releasing results.

"They need to look at their procedures," Jacobs said. "Not USADA so much as the sports federations" who leak the positive tests.

"They always talk about holding athletes to the highest standards," he said. "They need to follow their own rules. This kind of calls them on the carpet."