This was before Wilson went public, but he is undoubtedly the "former ambassador to Africa" discussed.
...the tale begins at the end of 2001, when third-rate forged documents turned up in West Africa purporting to show the sale by Niger to Iraq of tons of "yellowcake" uranium. Italy's intelligence service obtained the documents and shared them with British spooks, who passed them on to Washington. Mr. Cheney's office got wind of this and asked the C.I.A. to investigate.
The agency chose a former ambassador to Africa to undertake the mission, and that person flew to Niamey, Niger, in the last week of February 2002. This envoy spent one week in Niger, staying at the Sofitel and discussing his findings with the U.S. ambassador to Niger, and then flew back to Washington via Paris.
Immediately upon his return, in early March 2002, this senior envoy briefed the C.I.A. and State Department and reported that the documents were bogus, for two main reasons. First, the documents seemed phony on their face — for example, the Niger minister of energy and mines who had signed them had left that position years earlier. Second, an examination of Niger's uranium industry showed that an international consortium controls the yellowcake closely, so the Niger government does not have any yellowcake to sell. http://www.globalpolicy.org/security/issues/iraq/unmovic/2003/0613denial.htmFurther on, Kristof says "My understanding is that while Director of Central Intelligence George Tenet may not have told Mr. Bush that the Niger documents were forged, lower C.I.A. officials did tell both the vice president's office and National Security Council staff members."
In any case, the fact that they were forged did not require an investigation. The bogus signature is prima facia evidence of that fact.