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After the billionaire Edgar M. Bronfman Sr. told Forbes magazine that the self-help group Nxivm was a cult, some of its members came up with a plan to hack into his computer and spy on his emails, according to one of the people who took part in that effort.

A witness in the racketeering and sex trafficking trial of the Nxivm’s leader, Keith Raniere, described on Tuesday the group’s efforts to keep tabs on Mr. Bronfman, who died in 2013. They also monitored other perceived enemies, the witness said.

After emails to Mr. Bronfman containing spyware went unopened , his daughter, Clare Bronfman, a high-ranking Nxivm member, installed the program directly on her father’s computer, said the witness — whom prosecutors identified only by her first name, Daniela.

Soon, Daniela said, she was able to figure out the password to Mr. Bronfman’s AOL account.

“I would read all of his correspondence,” Daniela testified. “I would methodically check email by email.”