Kabul, Afghanistan ? The Afghan capital’s chaotic traffic and a tip from foreign intelligence foiled would-be suicide car-bombers who were planning a blast “beyond comprehension,” an Afghan intelligence official said Tuesday.

In the plot that failed Monday, the driver collided with another vehicle about 300 yards from the U.S. Embassy. The driver fled and police finally cornered his car at a checkpoint, where officers found it packed with a half-ton of explosives, said intelligence official Amonullah Barakzai.

Explosive materials and an electric igniting device are shown in a Toyota car in downtown Kabul, Afghanistan. Afghan officials on Tuesday gave more details on how a suicide bombing was foiled Monday.

Investigators focused on one unidentified suspect said to be a foreigner and a member of the al-Qaida terror group.

“He was on a suicide mission,” Barakzai said. “It was beyond comprehension.”

An alleged Afghan accomplice, also unidentified, was arrested with him, and a second Afghan jumped out of the car and escaped during the chase, said Gen. Din Mohammad Jurat, chief of the Interior Ministry police.

The traffic accident occurred in the heart of Kabul, within minutes of potential targets the fortress-like American mission, President Hamid Karzai’s palace offices, and the headquarters of the international security force that patrols the capital.

Jurat said interrogation of the suspects indicated their preferred target was an Afghan leader, and the International Security Assistance Force (ISAF) base was the fallback.

The alleged car-bombing plot was the latest sign of the insecurity unnerving the Afghan capital, coming just three weeks after a Karzai vice president was assassinated. Kabul is on guard against reprisal missions mounted by the ousted Taliban and al-Qaida, and is tense with ethnic friction among victorious anti-Taliban groups.

Investigators determined the car-bomb mission originated or passed through the troubled southeastern Afghan areas of Khost and Gardez, Jurat said. A statement by the national Intelligence Service said, without citing evidence, that the plan was developed abroad.

Foreign “friends,” including ISAF, had tipped the Afghans that a foreigner with explosives would be headed for Kabul, Barakzai said. Government agents tailed the car from the western side of the city, Jurat said.

Near the government printing center on Microyan Road, where aging taxis, lumbering trucks and sport utility vehicles dodge and weave for position, the suspects’ Toyota Corolla collided with a Toyota Land Cruiser, Barakzai said.

“He didn’t stop and drove off at high speed from the accident,” he said. “A mobile patrol from the Kabul security office was nearby and drove after him.” A half-mile away, the Corolla came to a crowded checkpoint and stopped, and the officers rushed the car and seized the two men.

Officers quickly became suspicious of the unusually heavy car doors, dismantled them, and found 1,100 to 1,300 pounds of TNT and C-4 explosives inside, rigged with an electrical detonating system, the intelligence official said. The explosives were overlaid with nails and bits of metal shrapnel that would have caused devastating injuries, said Jurat, the police general.

The car appeared to be speeding back in the direction of Gardez and Khost after the accident, Barakzai said. Taliban and al-Qaida holdouts moved through those southeastern areas in the aftermath of the U.S.-led military campaign that toppled the Taliban regime last December. Some are now known to be in nearby border areas of Pakistan.