Journalists can act as 'observers' reporting only what they see as the objective facts.
) comma between 'observers' and 'reporting'. If there were a comma, the nonfinite clause would be your adverbial, modifying 'act'; without the comma, it is a nonfinite clause acting as a modifier of 'observers'. There is also the option of considering the clause as a modifier of the whole sentence.
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I believe that the secret lies in the (omitted?) comma between 'observers' and 'reporting'.
If there were a comma, the nonfinite clause would be your adverbial, modifying 'act'; without the comma, it is a nonfinite clause acting as a modifier of 'observers'.
Journalists can act as 'observers' reporting only what they see as the objective facts.