"He says he wanted to go to heaven by killing himself and also killing infidels and supporters of infidels in Afghanistan," Amrullah Saleh said.
Investigators still have not established the identity or nationality of the suspect, captured Monday, but they know that the "very sophisticated" car bomb, almost a half-ton of explosives, was put together outside Afghanistan, Saleh told The Associated Press.
The Afghan official implied the alleged plot originated in Pakistan, where many al Qaeda figures are believed to have taken refuge since the collapse of Taliban rule last year.
The thickly bearded suspect has told interrogators the names of those who "motivated" him, Saleh said, and other people are being sought. He would not discuss the details of continuing security operations, however.
The capture of the alleged suicide terrorist had sent a chill through the city, especially since the shrapnel-packed Toyota Corolla had penetrated to a spot just hundreds of yards from the U.S. Embassy, Karzai's offices and the headquarters of the international security force patrolling Kabul.
It was the latest incident in a series — including the assassination this month of Vice President Abdul Qadir — that have put the Afghan capital on edge in the months since a U.S.-led campaign ousted the Taliban government and scattered the Afghan-based al Qaeda terror network.
Although Saleh, director of the intelligence service's office for Kabul, would not discuss other detained suspects, the Afghan Interior Ministry has reported that an alleged Afghan accomplice, also unidentified, is in custody. Saleh said his agency held only one, "the foreigner."
Afghan authorities said the explosives-laden automobile collided with a sport utility vehicle on Kabul's Microrayon Road late Monday morning, around the corner from the U.S. Embassy. When the Toyota sped off from the scene, suspicious security officers in the area gave chase.
During the pursuit, one of two Afghans in the car jumped out and escaped, said the chief of Interior Ministry police, Gen. Din Mohammad Jurat.
The car was rushed by officers when it had to stop at a crowded checkpoint about a half-mile from the accident scene. The car doors were found to be unusually heavy and were stripped, exposing bricks of C-4 explosives.
Earlier Wednesday, Maj. Angela Herbert, a spokeswoman for the International Security Assistance Force (ISAF) that patrols Kabul, said "valuable information" that ISAF shared a week ago with the Afghans may have contributed to foiling the plot.
Afghan intelligence officials said the information led them to deploy checkpoints and extra patrols in the city. But neither the Afghans nor the ISAF spokeswoman would elaborate.
Saleh said the bomb-rigged car originally was driven to the southeastern Afghan town of Khost and turned over to the foreign suspect. Khost lies 20 miles from the Pakistani border.
"He has admitted he worked for al Qaeda," the Afghan official said. "He has admitted his first target — he was tasked to kill His Excellency Mr. Karzai." Failing that, he said, the bomber was to target government ministers or foreign compounds or facilities in Kabul.
Saleh displayed photographs of the makings of the car bomb - yellow bricks of what he said was C-4 explosives, tubes that he said carried a liquid explosive, heavy batteries and connections, and two "last buttons" fitted by the gearshift to detonate the car.
Clearly it took experienced experts to fashion the bomb-on-wheels, he said. "They put a lot of thought into it."
By Charles J. Hanley