U.S. officials have recommended immunity for a former Mexican president implicated in a U.S. federal court case over his alleged role in a 1997 massacre.

Survivors of the attack and heirs of the 45 villagers killed in the massacre filed a civil lawsuit last year, accusing former Mexican President Ernesto Zedillo of crimes against humanity.

A court filing from the U.S. government Friday supported Zedillo's argument that he should be immune from prosecution. Lawyers for plaintiffs in responded Saturday by stating they understand the former president now has immunity as a result, thereby preventing them from proceeding with their case against him.

"The alleged actions as set forth in the complaint are predicated on former President Zedillo's actions as president of Mexico, thus involving the exercise of his powers of office," the U.S. government court filing said.

The filing concluded by saying "the United States has determined that former President Zedillo enjoys immunity from this lawsuit."

Arguments that Zedillo "should be held liable for lower level officials' tortuous conduct simply by virtue of his position as president at the time" were not enough to change the U.S. State Department's assessment, officials said in the filing.

The lead attorneys representing plaintiffs in the case said they were "deeply disappointed by the U.S. State Department's decision," which they said will "prevent us from ... presenting proof of (Zedillo's) responsibility for the 1997 Acteal Massacre."

"Based on the substantial testimony and documentation that we have collected, we are extremely confident that the highest levels of Mr. Zedillo's government had advance knowledge of and directly participated in the conspiracy that led to Acteal Massacre and its cover-up," Florida-based lawyers Roger Kobert and Marc Pugliese said, according to a statement released Saturday.

The attorneys also criticized the U.S. government for basing its decision "on communications between Mr. Zedillo, the current Mexican government and the U.S. government" that were not related to the plaintiff's lawyers or the general public.

Zedillo himself did not immediately respond to requests for comment.