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>> reporter: because that answer didn't quite square with the speeches, the promotional videos and the under writing of his pbs specials, cnn called the ben carson campaign asking for an interview. the response from the campaign's business manager quote, this will never happen. you are up to no good. we don't trust you. last week, we approached the candidate himself at a book signing in florida. dr. carson -- without any hesitation we were invited on his because for a two-minute interview. >> we're here to talk about a subject your campaign manager says you don't want to talk about, this manna...

And whenever anybody here falls short of that standard, to me, that's disappointing. >> reporter: dr. carson was asked about his relationship with mannatech in the recent cnbc debate. >> well, that's easy to answer. i didn't have an involvement with them. that is total propaganda. i did a couple speeches for them. i did speeches for other people. it's absolutely absurd to say i had any kind of relationship with them.

Carson's image was front and center on mannatech's website and promotional material. >> this is ray robins with the platinum group. we are proud to be the exclusive sponsors of the program you're about to watch with dr. ben carson. >> reporter: distributors calling them sevens the platinum group and signed on to underwrite ben carson's public television special. cnn spoke with three former sales and marketing insiders with mannatech that say the involvement but the neurosurgeon went a lodge way toward legitimizing a product were scientific committee was calling worthless. >> a neuro scientist ...

With them. that is total propaganda. i did a couple speeches for them. i did speeches for other people. it's absolutely absurd to say i had any kind of relationship with them. >> reporter: because that answer didn't quite square with the speeches, the promotional videos and the under writing of his pbs specials, cnn called the ben carson campaign asking for an interview. the response from the campaign's business manager quote, this will never happen. you are up to no good. we don't trust you. last week, we approached the candidate himself at a book signing in florida. dr. carson -- without any...

Relationship was in a company selling a product that the texas attorney general says is sham product? >> well, remember, they contracted me to give a speech. i do not, you know, go into great depth when i get a contract to do a speech. i go and do the speech. and i happen to like their product since i've been taking it, i almost never get sick but, you know, i constantly warn them not to use me as an endorser and some of the associates took the tapes and put them up on the websites and stuff like that. there is nothing i can do to control that. >> reporter: weren't you concerned at the time, t...