What is Nascimento testifying against?
The trial at the end shows Col. Nascimento in court testifying against corrupt politicians and bringing some of them down, when in reality he never actually witnessed any illegal activities by the politicians, so he is not a witness. It's just conjecture and accusations he cannot actually prove.
The only (weak) proof that ties them to the crimes is the phone tap which he had already passed to Fraga, so his contribution to the investigation ended there. Conjecture coming from a high ranking official would surely sell a lot of newspapers but would not be allowed in a court of law.
Good movie but the last 5 minutes try to tie things up nicely with this "sort of" happy ending.