The group's muted response, its refutation of early claims of responsibility attributed to a Taliban spokesman, and various accusations being tossed about, add to what is developing as a genuine whodunit.
What is known is that the attack was up close and personal. The 70-year-old Rabbani, who headed the High Peace Council established by the president's office to facilitate negotiations with the Taliban, was killed in his home upon receiving a man bearing a "special message" from Afghan Taliban leader Mullah Omar.
In a news conference held on September 22 by the country's intelligence service, the National Directorate of Security (NDS), an eyewitness to the assassination described the scene.
Ramatullah Wahidyar, a member of the High Peace Council and former deputy minister in the former Taliban regime, was the one who brought the bomber to meet with Rabbani and with top presidential adviser Mohammad Masoom Stanekzai, who was seriously injured in the attack.
"When we entered into the main building of [Rabbani's house, his secretary] Haji Nazir opened the door. The [suicide bomber] entered the room before me," Wahidyar said. "Ustad [Rabbani] was sitting right in front and [Masum] Stankezai was sitting next to him. Ustad then got up from his seat and opened his arms to embrace him, saying, 'Welcome welcome.' The guest then moved forward and bowed his head to give him a hug. Then there was a loud bang and I blacked out. When I regained consciousness, I saw that Ustad and Stanekzai had been taken away."
'It Was A Trick'
According to President Hamid Karzai, it was Stanekzai who first alerted him that a messenger had arrived in Kabul bearing an audio CD with a message of peace from a Taliban representative.
After listening to the audio message, the president said, he spoke with Rabbani, who then rushed back from a trip to Iran to listen to the recording himself.
"Before I went to the UN General Assembly, I knew that a meeting between Rabbani and Taliban members was going to take place," Karzai said.
But "it was not a peace message," Karzai said on September 22. "It was a trick."
Several questions remain unanswered.
What was on the CD and why would Rabbani drop his guard to meet with a supposed representative of the Taliban, which had threatened him on numerous occasions?
And why was the Taliban reaction so confused? If the assassination was intended to signal the Taliban's strong lack of interest in negotiations, then that is not the message that was sent with its uncharacteristic "no comment" issued on September 21.
Initially, Taliban spokesman Zabihullah Mujahid claimed responsibility for the attack in an interview with Reuters. But the same spokesman later wrote in a statement to RFE/RL's Radio Free Afghanistan on September 21 that the group "did not know about the incident."
Later the same day, the Taliban issued a broad statement to the media stating that it had no comment on Rabbani's assassination, rejecting the Reuters report as baseless, and demanding a correction. (Read a Twitter timeline of the back and forth at the end of this story.)
Indication Of Division?
On September 21, Reuters and other news agencies published follow-up reports highlighting the confusion and exploring the question of whether it is an indication of division within the highest ranks of the Taliban. On September 22, RFE/RL's Radio Free Afghanistan reported that Mujahid had sent a new message providing new contact details.
The circumstances and details, then, continue to be murky. But even without the clarity that the expected admission from the Taliban would provide, it is clear that the group is considered the main suspect.
"The perpetrators are known. It was the Taliban. But the Taliban doesn't have the ability to carry out such a well-organized operation alone," Balkh Province Governor Ata Muhammad Noor told Radio Free Afghanistan on September 21. "Pakistan's ISI (Inter-Services Intelligence) was supporting them."
Officially, however, investigators are allowing for other possibilities.
Speaking to journalists in Kabul on September 22, NDS spokesman Shafiqullah Taheri said that it is possible that apart from the Quetta Shura, the leadership council of the Afghan Taliban that is believed to be based in Pakistan, other elements were involved in planning and pulling off the attack.
"Our investigations are continuing. What we know so far is that a person named Hamidullah Akhund came to talk to the peace council," he said. "He claimed to be representing the Quetta Shura. This means that the Quetta Shura was involved in the attack. But as the investigations progress, we will know who else was involved."
There are a number of entities who could be seen as benefitting from the new landscape. While Rabbani's assassination is seen as a blow to the peace process, there have also been whispers that his warlord past was a hurdle to negotiations.
Whether it carried out the attack or not, observers predict that the Taliban's recruiting efforts and image as a hard-hitting insurgent group will be strengthened following the assassination.
Depending on how the chips fall, politicians opposed to both Karzai and the Taliban stand to gain. An opposition grouping headed by opposition leader Abdullah Abdullah has strongly opposed the idea of negotiating with the Taliban and has characterized the group as unworthy of participating in the government. This position could prove attractive in future elections.
And Rabbani did not lack for enemies. This was a man, after all, who is remembered by many Kabulis as the man responsible for the shelling of their city during the Afghan civil war.
So it is plausible that someone other than the Taliban might have wanted him dead. The question is, if not the Taliban, then who?
RFE/RL correspondent Frud Bezhan and Radio Free Afghanistan correspondent Zarghona Mangal contributed to this report