UPDATE:
Pamela Yates kindly sends the link to her investigation report:

What our guides from Batzul, victims of the massacre, asked of us is that we make clear that the guerrillas and not the Army carried it out. We intend to make a correction that will clarify what happened in this scene in both “When the Mountains Tremble” and “Granito”. It stands as a reminder of the terrible human costs of the violence in 1982-83, when the Guatemalan Government launched a massive offensive against the Maya Ixil people – part of a state sponsored campaign targeting civilians and which led to the CEH’s findings that during the years of the internal armed conflict 93% of the deaths were at the hands of the Armed Forces. What we have learned from this investigation will inform our new film “500 Years”. We remain committed to historical accuracy in our work and to supporting efforts to secure full human rights for all the people of Guatemala within a freely functioning democracy.

I applaud her efforts and thank her for contacting me.

(Please read also Gringo’s comments below.)

Earlier post:

Filmmaker to correct 1983 film on Guatemala war (emphasis added):

“When the Mountains Tremble” was an award-winning movie that awakened wide attention to the war in Guatemala. But at least one thing turned out to be wrong — and filmmaker Pamela Yates says she’s going to set it right.

A dramatic scene from the 1983 documentary will be corrected to show that the Batzul massacre highlighted in the film was committed not by the military, but by leftist rebels disguised as soldiers.
. . .
She did not specify how the films will be corrected. In an emailed message, she said “at this point it is premature to say just how I will modify the earlier films.”

Specifically, the Ejército Guerrillero de los Pobres – EGP, (Guerrilla Army of the Poor) were responsible for the Batzul massacre.

When The Mountains Tremble has a 20th anniversary edition; here’s the Amazon product description,

The film that shook audiences and critics alike upon its original theatrical release this revoutionary [sic] tour-de-force and Sundance Film Festival winner is now available for the first time on DVD. Digitally remastered to commemorate its 20th Anniversary this special edition chronicles the astonishing story of one woman who stood up for her people and helped wage a rebellion in the wake of seemingly unconquerable oppression. Shot at the height of a heated battle betwwen [sic] the heavily-armed Guatemalan Military and a nearly defenseless Mayan population filmmakers Pamela Yates and Newton Thomas Sigel threw themselves into the center of a storm to capture live combat footage with a surprisingly robust passion and exhilarating flair. As the first film to depict this previously unreported war it is firmly anchored by the firsthand accounds [sic] of Rigoberta Mench+Ý [Menchú] a Quich+ª [Quiche] Indian woman known around the world for her humanitarian efforts. Throughout the imminent chaos and danger Menchu provides courage and optimism in a time where death squads kill without conscience and an oppressive dictator seizes power. Updated after Mench+Ý [Menchú] was awarded the Nobel Peach [sic, Peace] Price WHEN THE MOUNTAINS TREMBLE includes a compelling filmmaker commentary as well as a never-before-seen introduction from Susan Sarandon and an illuminating epilogue reflecting on the country’s events a decade later. DVD Features: Filmmaker Commentary from Pamela Yates Newton Thomas Sigel and Editor Peter Kinoy; Never-Before-Seen Introduction by Susan Sarandon; Epilogue featuring Rigoberta Mench+Ý [Menchú]; Filmmaker Biographies; Interactive Menus; Scene Selection

Regarding Menchú, you may want to read The Truth About Rigoberta Menchú, a review of the book Rigoberta Menchu And The Story Of All Poor Guatemalans

Stoll went on to examine other aspects of Rigoberta’s book and soon found other claims that were not true. In her book, Rigoberta describes herself as an uneducated peasant girl. In interviews with Rigoberta’s relatives and former classmates, however, Stoll discovered that she’d spent several years at convent schools—first at the Colegio Belga in Guatemala City and then at the Colegio Básico Nuestro Señor de Candelaria in Chiantla, Huehuetenango, where she finished seventh grade—a remarkably high level of education for an Indian girl in Guatemala. Because she’d been in convent school, moreover, Stoll argues that Rigoberta can’t have been employed—as she claims to have been—as a maid for a rich family in Guatemala City, and can’t have worked in abusive conditions on coastal plantations—where she claims a younger brother Nicolás died of malnutrition. Stoll, in fact, found a living brother, Nicolás, who successfully resettled the family’s land long after the war had finished.
…
Stoll does not deny that Rigoberta’s village was destroyed and that half her family was killed, including her father, her mother, and her brother Petrocinio. But he points out that many of the other events in Rigoberta’s book are either distorted, fabricated, or claim to be eyewitness accounts of events which Rigoberta herself cannot actually have seen. The reason for all this, Stoll argues, is that after Rigoberta fled to Mexico in 1980, she allied herself with guerrilla groups there and “drastically revised the prewar experience of her village to suit the needs of the revolutionary organization she had joined.” In other words, when she wrote her book, Rigoberta was essentially serving as a propagandist.

Last Friday, Fermin Felipe Solano Barillas, also known as ‘Teniente David’, was sentenced to

90 years in prison for ordering a group of 10 guerrillas to strangle and kill in the town of El Aguacate 22 pro-government indigenous farmers, accusing them of collaborating with the army

Solano was with the Organización Revolucionaria del Pueblo en Armas, or ORPA (Revolutionary Organization of Armed People), another one of the four guerilla groups of the Unidad Revolucionaria Nacional Guatemalteca or URNG (Guatemalan National Revolutionary Unity).

The correction of When The Mountains Tremble is quite overdue, but in LatAm leftist circles the truth is glimpsed at a very slow pace.