A would-be homicide bomber with more than a half-ton of explosives packed into his car was just 300 yards from the U.S. Embassy on Monday when he was stopped by a chance traffic accident, a top Afghan intelligence official reported Tuesday.

Foreign intelligence information indicated the alleged terrorist, who was captured by Afghan authorities after a car chase through Kabul, is a foreigner and a member of the Al Qaeda terror group, said Amonullah Barakzai, deputy director of the Afghan intelligence service.

"He was on a suicide mission," Barakzai said. But he said Afghan authorities were not sure whether his target was the embassy, other foreign installations or the Afghan leadership, including President Hamid Karzai, whose palace offices are about a half-mile from the accident scene.

An Interior Ministry spokesman said late Monday a second unidentified man also had been arrested in the case. But Barakzai reported only the single arrest, and said investigators had not determined the man's identity or nationality.

"Unfortunately for the past 24 hours we have not been able to make him speak," he said.

The Kabul government sent additional troops into the capital's streets late Monday to guard against new attacks.

The alleged car-bombing attempt was the latest development feeding concern about security in Kabul in the wake of the killing of Vice President Abdul Qadir three weeks ago. A Turkish-led international security force patrols Kabul as foreigners train a new Afghan national army, police force and bodyguard contingent to protect the Afghan Cabinet.

A spokeswoman for the international security force, British Maj. Angela Herbert, said Tuesday she had no information to offer on the incident, and added, "Nobody can confirm either way who the target was."

A government television report Monday night on the arrest was accompanied by video of the man's Toyota Corolla, whose door panels were exposed to show yellow blocks of what the intelligence service statement said was explosives.

The report also showed a photo of the suspect, a heavily bearded young man in a waistcoat and open shirt. His dress and appearance seemed Afghan or Pakistani, but the government statement read by the station said only that he was a foreigner and the plan was developed abroad.

Barakzai said the Corolla collided with a Toyota Land Cruiser late Monday morning along Microyan Road by the government printing center, around the corner from the sprawling, heavily guarded U.S. Embassy. A tall steel outer wall was erected at the embassy four months ago.

"He didn't stop and drove off at high speed from the accident," Barakzai said. "A mobile patrol from the Kabul security office was nearby and drove after him."

When the fugitive had to stop at a crowded road checkpoint a half-mile away, the officers rushed the car.

They became suspicious of the car doors, which were very heavy. Dismantling them, they found "500 or 600 kilograms" — 1,100 to 1,300 pounds — of TNT and C-4 explosives, rigged with an electrical detonating system, the intelligence official said.

The government TV report late Monday said the suspect was injured in the accident and was taken to the intelligence service's offices for interrogation.

Afghan authorities had been tipped by information from the International Security Assistance Force and "other organizations" that a foreigner with explosives would be entering Kabul, Barakzai said, and so had deployed special mobile patrols and set up checkpoints.

The suspect had driven from the Khost or Gardez area of southeastern Afghanistan, and appeared to be speeding back in that direction after the accident, Barakzai said. Taliban and Al Qaeda holdouts moved through those areas in the aftermath of the U.S.-led military campaign that ousted the Taliban regime last December. Some are now known to be in nearby border areas of Pakistan.

Karzai's transitional government has been beset by worries of a Taliban resurgence, terrorism by Al Qaeda and ethnic tensions among the victorious anti-Taliban groups, particularly Defense Minister Mohammad Qasim Fahim's northern Tajiks and the Pashtuns of President Karzai.

Qadir, who had become one of three vice presidents in June, was the second government minister to be killed this year. The aviation and tourism minister, Abdul Rahman, was beaten to death in February at Kabul airport. Neither killing has been solved.

Last week, because of rising security fears following the Qadir killing, Karzai sidelined his Afghan bodyguards and called in U.S. troops to replace them. Diplomats said the move followed "serious threats" against Karzai, some believed to have come from within his own Cabinet.