Amrullah Saleh told Parliament on Monday that the plot to kill Karzai was hatched last month and the gunmen had rented the room they opened fire from 45 days before the attack.
Karzai and other dignitaries escaped unharmed from Sunday's assault launched during a ceremony in Kabul.
The Taliban claimed responsibility.
Suicide Attack Kills 15 In Eastern Afghanistan
A NATO spokesman says a suicide attack in eastern Afghanistan has killed 15 local people and wounded 25.
Maj. Martin O'Donnell said there were NATO troops in the area of the attack Tuesday but there were no alliance casualties.
He said the militants opened fire before a suicide bomber blew himself up in Khogyani district of Nangahar province.
Alliance medics were treating the wounded.