KABUL — Suicide bombers riding in minivans struck the headquarters of the Afghan intelligence agency Wednesday, detonating a powerful car bomb and raising questions about how insurgents could pull off such a bold attack in one of the most heavily guarded areas of the city.

The explosion went off around noon at one of the gates of the National Directorate of Security around the site. The attack, which was followed by sporadic gunfire, killed one employee of the agency.