World Anti-Doping Agency director-general David Howman says Lance Armstrong pursued what appears to be a systematic doping program for a decade "probably with the knowledge" of people who were charged with detecting drug cheats.

Howman told New Zealand's LiveSport Radio on Friday that Armstrong's repeated claim he has never tested positive for a banned substance could no longer be regarded as proof of his innocence.

"What seems to have happened in this particular scenario is that it went on for many years under the noses of those who were supposed to be detecting it and at times probably with their knowledge," he said.

Howman said Armstrong had finally been caught because fellow cyclists had broken a code of omerta and confessed their parts in a "conspiracy to defraud the sport".

He did not specifically identify the agencies or individuals he suspected may have turned a blind eye to doping by Armstrong or his teams. But he referred to "suggestions" contained in the report of the United States Anti-Doping Agency that irregularities in some Armstrong tests might not have been investigated as rigorously as they should have been.

Howman said failed doping tests were no longer the benchmark needed to identify cheats.

He used the case of Olympic track and field athlete Marion Jones to illustrate flaws in the system. Once considered the fastest woman in the world, Jones served prison time and gave back her five medals from the 2000 Sydney Olympics after being convicted of lying to federal investigators about using performance-enhancing drugs.

"Marion Jones didn't fail a test either and she competed for seven years, won all sorts of gold medals and world championships, was tested about 160 times and none of them was positive," he said.

"Lance Armstrong has (been) tested more than a couple of hundred times. None of them were positive in terms of recorded positives. There have been suggestions that there may have been a couple of them that were not fully investigated so they remain at the moment suggestions.

"What we have to say is the science can catch some but it doesn't catch them all. What we've been talking about over the last few years is that where there is evidence gathered in relation to cheating - and it can be gathered by the police or by customs or by other people - then it should also be brought together to sheet home a sanction process against the cheating athlete. That's what they've done in this case."

Howman said the result of USADA's investigation of Armstrong was a "compelling" case against the seven-time Tour de France winner.

"It reads like 10 years of systematic doping and that's been through the mill of sworn testimony so if anyone is found out to have lied under oath they can face charges of perjury - and in the United States you can go to prison for that," he said.

"This is a very compelling series of informants who have given evidence, trying to help their sport get clean and what they've done is broken the (code of silence) to do that and I suppose that's taken quite a bit of courage."

In an earlier statement, WADA president John Fahey welcomed USADA's "reasoned decision" in the Armstrong case. But the statement said it would be inappropriate to comment further until the UCI had decided whether it would take the matter to appeal. It had 21 days to do so.