As of July, 1981, a party not owning a patent may file an ex parte request for re-examination.

35 U.S.C. §301 Citation of prior art.
Any person at any time may cite to the Office in writing prior art consisting of patents or printed publications which that person believes to have a bearing on the patentability of any claim of a particular patent. If the person explains in writing the pertinency and manner of applying such prior art to at least one claim of the patent, the citation of such prior art and the explanation thereof will become a part of the official file of the patent. At the written request of the person citing the prior art, his or her identity will be excluded from the patent file and kept confidential.

37 USC (A)(1)(A) - Rules of Practice in Patent Cases
§1.510 Request for ex parte reexamination
(a) Any person may, at any time during the period of enforceability of a patent, file a request for an ex parte reexamination by the Office of any claim of the patent on the basis of prior art patents or printed publications cited under § 1.501.

The press glibly reports patent office actions in re-exams as a victory or shift in momentum, ignoring the reality that a reply by the patentee may enlighten the patent office that its rejections were incorrect.

Ex parte requests for re-examination are the last hope for an otherwise lost patent litigation defendant. Requests for re-exams are typically made only after defense attorneys fail to prevail in court. The reason is risk.

If the patent office allows a patent through re-examination, it greatly strengthens presumption of validity, which weakens any subsequent invalidity attack in litigation. Further, an external party has only one shot in requesting re-examination, only one chance to submit documentation influencing the patent office. Before the court during litigation, defense attorneys have multiple tosses at the bulls-eye, being able to file a series of briefs that refine arguments or smudge previous errors in argument.

So, the least-risk defensive strategy is to fight the patent dragon before the court, and then make one final thrust for re-examination redemption after being burnt. But there is no denying that the defense misfired in not being able to get their invalidity story across to the court, and thus having to resort to a ex parte re-examination request after being fried for infringement.