Agent Vreede is a bad guy Edit
He is seen in his flashforward as leaving the building, so he might have let the gunmen in.
Also, he has been "assisting" Olivia follow the lead on Gabriel wich has yielded some intriguing clues. However not one scene has had him report any of these findings either to Mark or anyone else at the Mosaic Investigation. One could believe he may have reported off-screen but given the fact Olivia is closely involved, wouldn't Mark have talked to her by now? Either Vreed is truly withholding information from the bureau or someone has droped the ball, storywhise.