Hamidullah Akhund was arrested "somewhere in Kabul in the past few days", New York Times reported, quoting an Afghan government spokesman Sifatullah Safi.
Few details were released but it was alleged that he was connected to the Taliban militants who sent the suicide bomber to kill Rabbani, chairman of the Afghan Peace Council, the NYT reports.
The killer, identified as Mullah Esmatullah, was escorted into Rabbani's heavily guarded home last Tuesday after pretending to carry a peace message from the Taliban, the New York Times reports.
It is alleged that Akhund was a critical interlocutor between the peace council and Taliban leaders in Quetta, Pakistan, near the Afghan border.
Akhund traveled to Kabul twice to meet with Rabbani - as long as four months ago - and another member of the High Peace Council, Masoom Stanekzai, according to an account given at a news conference this week by Ramatullah Wahidyar, a former Taliban member who now serves on the council, NYT reports.
Akhund recorded an audio message from Rabbani to the Taliban, which he promised to deliver, and passed along reports of his progress and conversations with high-ranking Taliban leaders, Mr. Wahidyar said.
About a week ago, he said, Akhund called with news of what seemed like a breakthrough: the Quetta group was ready to talk to the Afghan government, and had an important message to send.
Akhund said he could not go to Kabul personally, but would send a man he trusted, Mr. Wahidyar said. The substitute turned out to be the bomber.
Afghan President Hamid Karzai has appointed a high-level team headed by the defense minister to investigate the assassination, it reports.
"We are getting close to the truth," the spokesman said.
The killer claimed to be a peace emissary from the Taliban leadership, Hindustan Times reports.
Hindustan Times reports:
Police and other senior figures have blamed the Taliban for the attack, but unusually, the leaders of Afghanistan's ten-year insurgency have so far refused to comment on it.
The investigation will be led by Afghan defense minister Abdul Rahim Wardak, seen as close to Karzai, a statement from the presidential palace said.
A government official said that Akhund had spoken to the bomber a day before the attack. The bomber was believed to be a trusted Taliban emissary, had left the official guesthouse where he was staying to speak to the suspect.
President Karzai and his government have come under intense pressure from political rivals and the Afghan public to arrest whoever was responsible for the plot to assassinate Rabbani.