The U.S. State Department termed the shooting a terrorist act, and Shrum's parents rejected the idea that their son was there as a Christian missionary.
Jim and Lynda Shrum said their son's quest to help others out of poverty and build bridges between cultures began about a decade ago, when he took a college trip to India to help improve conditions at orphanages.
"He had a knack with people," Jim Shrum said. "Knocking down barriers, accepting other cultures, taking the good and the bad."
The Shrums also said they feared for the safety of Joel Shrum's widow, two young sons - ages 4 and 1 - and his Yemeni friends. They said they did not want to talk about his widow's plans out of concern for her safety.
Shrum's parents live in Harrisburg, about 20 miles from the town of Mount Joy, where Joel Shrum grew up.
Gayne Deshler, who coached Shrum and his younger brother, Justin - and, in the late 1970s, their father - on the football team at Donegal High School, said the family was close.
"All of them were just good kids," he said. "They were very coachable. They were very good students. They never got in trouble in school, always very polite."
Shrum, who graduated in 2000, was a team captain and made the first team all-league in his senior year for offense and defense.
Shrum moved to Yemen in 2009 with his wife, Janelle, and older son. He began working for the Swedish-run International Training Development Centre in Taiz in 2010, and eventually was promoted to deputy director, his parents said. His second son was born in Taiz.
A Protestant, Shrum was surrounded by Yemeni friends and deeply respected their Muslim faith. On a visit in 2010, his parents said, they met Joel and Janelle Shrum's friends and were struck by their warmth and hospitality.
"We could see that their friends loved them," Lynda Shrum said.
Taiz security director Ali al-Saidi said Monday that the investigation remained open. In Washington, State Department spokeswoman Victoria Nuland told reporters the United States was urging Yemeni authorities to bring those responsible to justice.
"We condemn this terrorist act in the strongest terms, and we express our deepest condolences to his family and his friends," Nuland said.
Karen Langley of the Pittsburgh Post-Gazette contributed to this article.