US anti-doping chief Travis Tygart has slammed Norwegian former pro rider Thor Hushovd for keeping silent after Lance Armstrong admitted doping to him in 2011.

Hushovd's autobiograohy, Thor, was published recently in Norwegian. In it, he tells of a lunch with Armstrong after the 2011 Tour of California. Armstrong's former team-mate Tyler Hamilton had recently admitted doping and accused Armstrong of being at the centre of doping at the US Postal Service team.

"We all did it," Armstrong told Hushovd.

But Hushovd subsequently gave non-committal answers when asked about Armstrong's doping.

Now Travis Tygart, the head of US Anti-Doping, has hit out at Hushovd for not coming forward with what he knew.

"You're no hero when you sit still without doing something about injustice against great athletes," Tygart told Norwegian news website VG.no. "It is especially difficult to [fight doping] when someone in the sport abusing his power by trying to attack the idea of​ a clean sport and clean athletes."

Hushovd maintains he rode clean for his whole career. Asked about doping at the launch of the book, he said: "It's not my job to clean up here."

Hushovd said that if he had gone to the anti-doping authorities in 2011, Armstrong and his friends would have destroyed his career.

Tygart wants to hear why Hushovd felt unable to speak.

"It's a sad day when athletes are afraid or unable to stand up for fair play and integrity in sport," he said. "Doping Norway and USADA would very much like to hear from him why he did not do anything, but instead sat quietly and let the lies and deception continue."

The World Anti-Doping Authority (WADA) has told cycle racing website CyclingTips that it does not consider Hushovd to have broken the anti-doping code by staying quiet.

Meanwhile Hushovd says anti-doping authorities should stop what he calls the "witch hint for old sinners" and concentrate on improving tests so they are harder to fool and evade.

Tygart is unimpressed.

He said: "It is the kind of thing you'd expect to hear from someone who sat still and let the deception continue. Those who heard these things, however, should have said so.

"Sport will only be cleaned up if those who have cheated in the past are held accountable for what they did. If you cheated before without getting caught, so the chances are very much greater that you will continue later, in the belief that you will never be discovered. That gives little hope for all of us who love sport."

Tygart admitted that he was not sure if information from Hushovd would have made USADA's pursuit of Armstrong any easier.

"Eleven former teammates of Lance showed enough courage to talk to us," he said. "It is difficult to be absolutely certain. I hear among other things, about an episode where Hushovd saw someone with a syringe in a hotel room.

"But we might have missed a great opportunity with Hushovd not talking at the time. He could have at least given some of those who talked more credibility when the storm was bad. I'm thinking of Betsy Andreu who for years claimed to have heard Armstrong admit doping in the mid 90s."