(CNN) -- 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' tortious 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.
The case has been closely watched on both sides of the border, particularly as the six-year term of current Mexican President Felipe Calderon -- whose administration has been accused by some of human rights abuses in the nation's drug war -- nears its end.
Zedillo was Mexico's president from 1994 to 2000.
The civil case against him was filed nearly a year ago in Connecticut, where Zedillo resides while he teaches at Yale University. It accuses him of being responsible for the killings of 45 villagers in the southern Mexican state of Chiapas, and afterward conspiring to cover it up.
Ten anonymous plaintiffs were seeking about $50 million in damages against him, according to court documents.
Although the killings happened in Mexico, the lawsuit argued that a long list of U.S. laws and international treaties -- including the Alien Tort Claims Act and Torture Victim Protection Act -- give the American federal court jurisdiction to try the case.
The killings occurred on December 22, 1997, and are considered among the more brutal incidents during an armed conflict that began three years earlier in southern Mexico after Zapatista rebels pushed for more rights for indigenous people.
Zedillo has denied allegations that his office was somehow responsible for the deaths.
"These anonymous accusations that President Zedillo was somehow complicit in the events in Acteal are baseless and outrageous," he said in a January court filing. "The December 1997 attack in Acteal was an appalling tragedy. But that tragedy was not the result of an elaborate conspiracy by the Mexican federal government, masterminded by President Zedillo."
CNN's David Ariosto and Mariano Castillo contributed to this report.