'as' - grammatical role?

[peteryoung]

Deputy Assistant Police Commander Brian Paddock told a news conference a review of data reveals the bombs on three subway trains exploded within seconds of each other, rather than over the course of 26 minutes, as originally believed.

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Is 'as' here a preposition, conjunction, pronoun, or adverb? It seems hard to decide.
Is it possible to substitute 'a fact' for 'as' without altering the original meaning much?

Thanks!

[Marylin]

Deputy Assistant Police Commander Brian Paddock told a news conference a review of data reveals the bombs on three subway trains exploded within seconds of each other, rather than over the course of 26 minutes, as originally believed.

---------
Is 'as' here a preposition, conjunction, pronoun, or adverb? It seems hard to decide.
Is it possible to substitute 'a fact' for 'as' without altering the original meaning much?

Thanks!

As functions as a conjunction and it does indeed bring in additional facts to the previous statement. There is a group of expressions that do that in case where facts are known to the reader as well as the speaker.

These are:
as believed
as discussed
as agreed upon
as suggested

There is no subject it after as and the verb is used in a passive form.

more examples:

I am sending you a check for $200 as previously discussed.
The meeting will take place at 2pm, as agreed.

[Tdol]

Conjunction IMO- you could expand the phrase into 'as they originally believed', which makes it clearer.

'A fact' doesn't work very well because it wasn't a fact- they were wrong. However, grammatically, it's OK.

[peteryoung]

It's so nice of you. Thank you both A^_^