A small group of FBI agents huddled outside a Home Depot in Atlanta in January 2006, watching a young man suspected of being linked to terrorists as he walked out the door with materials that could be used to make a bomb.

They knew Syed Haris Ahmed had researched bomb-making techniques online and shaved his head, as some jihadis have done before an attack. However, they had to make a quick decision faced by other terror investigators across the country: They could swoop in and keep him from potentially building a bomb, which could leave them with a case too weak to win a conviction. Or they could keep building a stronger case - and risk a potential terror attack.