Israeli criminal attorney Avigdor Feldman said he met with the man, dubbed "Prisoner X", a day before his death.
"I met with a balanced person, given the tragic outcome, who was rationally weighing his legal options," Feldman told Channel 10 Television.
He said the detainee was charged with "grave crimes" and that there were ongoing negotiations for a plea bargain. The attorney did not elaborate on the allegations, which he said the prisoner denied. Reporting in Israel on the case is still subject to strict government censorship.