MEXICO CITY — Emma Theissen Álvarez says she will never forget the faces of the three men who came to her Guatemala City house that day in 1981 looking for her daughter, a student leader who had escaped from her military captors. When they did not find their target, they grabbed her 14-year-old son, Marco Antonio, instead.

She never saw her boy again.

For years, the family said nothing, mute in its pain. But when Ms. Theissen and her three daughters finally went to court, helping secure the convictions of retired military commanders of crimes against humanity, they found a sort of healing.

“One feels that one is doing something to bring a little justice to Marco Antonio,” said Ms. Theissen, who is now 84.

Now that justice is in peril.

In a reversal that seemed unimaginable just a few months ago, Guatemalan lawmakers are moving forward with a proposal to grant amnesty for war crimes committed during the country’s brutal 36-year civil war.