Atlanta ? A small group of FBI agents huddled outside a Home Depot in Atlanta in January 2006, watching a young man suspected of terrorist ties as he bought 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. They had to make a decision faced by other terror investigators across the country: Swoop in and keep him from potentially building a bomb, which could leave them with a case too weak to win a conviction, or keep building a stronger case — and risk a potential terror attack.

“If we have a terrorist event, we could solve what happened. But that means we’ve lost,” said Rick Maxwell, an assistant special agent in charge in Atlanta.

They held off, relying on federal explosive experts who told them the PVC pipe was not enough to create a dangerous explosive. They also sent local police pretending to be on a routine call to question Ahmed about the piping, which they determined was being used for a school project.

Ahmed and co-defendant Ehsanul Islam Sadequee were ultimately convicted of terror-related charges earlier this year, and federal prosecutors said the case exemplified their strategy of snuffing out potential plots. Sadequee faces up to 60 years in prison, and Ahmed faces 15 years behind bars when the pair is sentenced today.

However, federal investigators have a mixed record when other terrorist plots are broken up early. Five Muslim immigrants were convicted last year of conspiring to kill U.S. soldiers at Fort Dix in New Jersey, but they were acquitted of attempted murder after prosecutors acknowledged they were likely months away from acting.

Meanwhile, the FBI has recently been criticized for not launching a deeper investigation into Maj. Nidal Hasan, who is accused of shooting and killing 13 people Nov. 5 at Fort Hood, Texas.