While he is not being tried for war crimes, the case is expected to include testimony from former members of the Guatemalan special forces and survivors of the conflict, including Ramirez.
Prosecutors declined to comment on the case.
Sosa's lawyer, Shashi Kewalramani, said the case isn't about what happened in Guatemala but whether his client knew he was accused of a crime when he applied to become an American.
"The only crime he has been charged with in the United States is lying on an application," he said.
Sosa sought asylum in the U.S. after fleeing Guatemala in 1985 and was denied, heading to Canada instead. He later obtained a green card after marrying a U.S. citizen, and applied to naturalize in 2007.
Sosa said he was in another town at the time of the Dos Erres massacre, helping build a school and organize sporting events.
In a letter published by the investigative journalism nonprofit ProPublica, Sosa said he disclosed his membership in the Guatemalan military when he sought asylum in the U.S.
"I desperately need people to know that I have NOT committed a crime nor do I have a criminal mind," Sosa wrote in the letter from a prison in Canada, where he is also a citizen. His brother, Hugo Sosa, verified the account.
If convicted of making a false statement and procuring naturalization unlawfully, Sosa also could face up to 15 years in prison. Federal prosecutors also want to strip Sosa of his American citizenship, something that could pave the way for his return to Guatemala.
A spokesman for Guatemalan prosecutors says they will seek to extradite Sosa to face charges for "crimes against humanity."
While the trial will bring Guatemala's gruesome past to an American courtroom, 50 miles away in Los Angeles, Peccerelli will be continuing his work to bring closure and answers to survivors.
He hopes Ramirez, who will also speak with community members about the DNA program, can help win the trust of Guatemalans still scarred by the war. Since learning his true identity, Ramirez met family he never knew he had — including his father, who was not in Dos Erres the day of the massacre — and obtained political asylum to stay legally in the U.S., something his lawyer, Scott Greathead, said might be a possibility for other survivors.
Associated Press writer Sonia Perez D. contributed to this report from Guatemala City.