But authorities don't know why the 20-year-old suspects escalated from offenses like break-ins and jewelry heists to allegedly killing their victims and burying them in two pits so deep beneath the ground that a backhoe and dozens of people were needed to sift through the dirt.
Police found the missing men after a grueling, five-day search in sweltering heat and pelting rain.
For Cosmo DiNardo, whose lawyer said he confessed to all four killings in exchange for being spared the death penalty, brushes with the law began in his early teenage years.
He was about 14 when the Bensalem Police Department first had contact with him. He had more than 30 run-ins with its officers over the next six years, department director Frederick Harran said, although court filings reflect only the minor infractions and traffic stops that came after age 18.
DiNardo enrolled at Arcadia University in Glenside in the fall of 2015 with hopes of studying biology and had an eye on international travel, according to a blog post announcing the incoming class.
"I'm going to go overseas, hopefully to Italy and the rest of Europe," he is quoted as saying.Read more at Associated Press