Paul Shay, a 64-year-old New York City plumber, and his wife, Monica, 58, had arrived the night before to spend the holiday weekend with Shay's nephew Joseph, 43; his girlfriend, Kathryn Erdmann, 37; and Erdmann's 2-year-old son, Gregory Bosco. Monica Shay, Joseph Shay, and the toddler died of their wounds. Paul Shay and Erdmann remain hospitalized.
Geisenheyner claimed to be seeking revenge against Paul Shay, who he said cheated him out of the proceeds of an insurance-fraud scheme the two had planned five years earlier. Authorities have not confirmed that story.
But while Geisenheyner had allegedly plotted his retribution for months, a handful of his friends maintain that his purported medical condition drove him to finally take action.
"I think he knew his life was over. He wasn't afraid to die," said Carlton Richardson, who grew up with Geisenheyner in the 1970s and stayed close throughout the years.
Richardson, 53, of Media, said in an interview last week that Geisenheyner had told him about his tumor shortly after coming out of prison in late 2009.
When his friend came to him July 3, hours after the Shay shootings, Richardson said, he couldn't help but wonder whether the condition had affected Geisenheyner's thinking.
Michael Madden, who taught Geisenheyner and Richardson at the Wallingford-Swarthmore Alternative School in the late 1970s, maintained that the pituitary condition might have affected Geisenheyner for longer.
He recalled Geisenheyner as a charismatic yet intense teen who liked to push boundaries in school but never exhibited malicious intent.
"Mark was a challenge," he said. "He was a pain . . . [for] a teacher. He wasn't someone who would just sit there and behave. "
Madden, now a therapeutic counselor and teacher at Pennsylvania State University's Delaware County Campus, said that when Geisenheyner told him about the pituitary tumor six months ago, he wondered how far such a condition might go toward explaining his adolescent behavior.
The pituitary gland - at the base of the brain - controls the secretion of hormones including those associated with impulse control. Tumors growing there can take years to become large enough to diagnose.
But whether or not the medical examiner's tests show Geisenheyner suffered from such a condition, Madden cautioned against blaming the man's actions on his diagnosis.
"There's just no one cause or explanation for this," he said.
Contact staff writer Jeremy Roebuck at 267-564-5218, firstname.lastname@example.org, or @inqmontco on Twitter. Read his blog, "MontCo Memo," at www.philly.com/montcomemo