Guerra got the news in a letter from the Marines that arrived at her Denver home on Aug. 14. Her son, Bob, was with her.
"I started crying because I knew it was about my brother," she said. "We were crying, we jumped, we hollered."
Guerra, now 71, was 15 when Jacques was born.
"He was a very loving, very caring -- well, he was my baby brother," she said. "He was just a really good person."
Jacques grew up in La Junta, a small town about 140 miles southeast of Denver. He joined the Marines in October 1974, shortly after his 18th birthday. His family was apprehensive but didn't try to dissuade him, Guerra said.
"It was something he wanted to do," Guerra said. "He wanted to go and serve his country and do his best."
Jacques died just seven months after enlisting.
Twelve of the 13 missing servicemen are now confirmed to have died, Parker said. She said she would not discuss the 13th because an investigation is ongoing.
The Mayaguez operation is considered the last U.S. military engagement in Southeast Asia after the long and bloody war in Vietnam. The last U.S. combat troops left South Vietnam in 1973, and the South Vietnamese capital fell to communist North Vietnam on April 30, 1975, just two weeks before the Mayaguez engagement.