An audio CD purportedly supplied by the Taliban's Quetta Shura leadership council in Pakistan to Afghan officials called Rabbani, a former Afghan president and head of Afghanistan's Peace Council, a "dear" and "respected" teacher, The New York Times reported.
But the contact who brought that CD killed Rabbani in a suicide bomb attack Tuesday.
"We mistook it as a peace message," Karzai told reporters at the presidential palace. "That was only deception."
Rahmatullah Wahidyar, the High Peace Council member who introduced the bomber to Rabbani and was injured in the attack, told reporters in Kabul the council believed it had made contact with the Quetta Shura through an "honest man among the Taliban."
But while the final man sent to meet Rabbani carried positive messages on a CD, he also brought a bomb hidden in his turban that detonated the moment the two men embraced.
Karzai said the messages on the CD were "amazing."
"There were lots of good messages in that CD," he said.
Wahidyar said there were some "motivating" things in the messages, including the Taliban's longstanding promise to return to Afghanistan after international forces leave. The message also said "there is lots of adultery in Afghanistan and asked what will the government do about this," Wahidyar said.
Karzai closed his remarks at the palace by asserting "peace is something that all the people in Afghanistan want."
Ryan Crocker, the U.S. ambassador to Afghanistan, issued a statement saying the attack "raises very serious questions as to whether the Taliban and those who support them have any real interest in reconciliation."