Germany's former foreign minister Joschka Fischer has accused the former head of the CIA George Tenet of making implausible claims about the handling of the Curveball case by the US.

On Wednesday Tenet, the director of central intelligence between 1997 and 2004, issued a statement on his website saying he discovered "too damn late" that Curveball – the Iraqi defector who became a key source for the CIA and the German secret service (BND) – might be a fabricator.

Reprinting an extract from his autobiography, Tenet claimed he only found out in 2005, two years after the Iraq invasion, that the BND had doubts about Curveball's claims to have witnessed first-hand Saddam Hussein's bio-weapons programme.

Asked by the Guardian whether Tenet's claims were plausible, Fischer said: "No. I don't think so."

Fischer said the BND realised some time before the war that Curveball was not a watertight source, and passed on his testimony to the CIA with warnings attached.

"Our position was always: [Curveball] might be right, but he might not be right. He could be a liar but he could be telling the truth," said Fischer at a press conference in Berlin to promote his memoir about the Iraq war.

Fischer said Germany was put in a "very difficult position" when the CIA asked whether they could "have" Curveball, or at least use his evidence to justify a war in Iraq. Germany's official position was that it would not join the coalition of the willing. Fischer himself famously told Donald Rumsfeld in February 2003 that he was "not convinced" about the case for war.

"On the one hand we didn't want to withhold from the US any bit of relevant information we had about possible WMD in Iraq. On the other hand, we did not want to take part in any propagandistic exploitation of material, which was far from proven, to justify a war," Fischer writes in his new autobiography, I Am Not Convinced.

He added: "We decided, therefore, that we would do our duty by sending the Americans all the information we had, together with our assessment that that information came from a deserter and that we had not verified or substantiated it ourselves, and that it could be completely wrong."

Fischer said today: "We, the German government, decided to pass on the evidence, and I think that was the right thing to do."

He said the then head of the BND, August Henning, wrote a letter to the CIA outlining the possible problems with Curveball. Fischer also pointed out that it was common practice in security circles – then, as now – to not rely on a single source, but to get at least three independent sources that corroborate each other.

Asked what he thought about Colin Powell demanding answers as to why the CIA and its military arm, the DIA, never told him about Curveball possibly being a liar, Fischer said he couldn't comment.

"[Powell] is a very good friend of mine, but we have never ever spoken about this phase," he said. "If there is something to discuss, he has to initiate the conversation."