Can examiners use prior art not cited in the reexamination request?

Yes, examiners are not limited to the prior art cited in the reexamination request. According to MPEP 2644:

“The examiner is not limited in making the determination based on the patents and printed publications relied upon in the request. The examiner can find ‘a substantial new question of patentability’ or ‘a reasonable likelihood that the requester will prevail’ based upon the prior art patents or printed publications relied upon in the request, a combination of the prior art relied upon in the request and other prior art found elsewhere, or based entirely on different patents or printed publications.”

This gives examiners flexibility to consider a wide range of prior art in their determinations.

To learn more:

Topics: MPEP 2600 - Optional Inter Partes Reexamination, MPEP 2644 - Prior Art On Which The Determination Is Based, Patent Law, Patent Procedure
Tags: Patent Examiner, Reexamination Request