It was August 2006 when one of the young Muslim men accused of plotting to kill soldiers at Fort Dix first broached the idea, according to the authorities. Talking to an informer who was secretly taping the exchange, the young man said that he thought he could round up six or seven other men willing to take part, and that a rocket-propelled grenade might be the most effective weapon, the authorities said.

And he had one more notion: He wanted the informer to lead the attack, according to a federal complaint. “I am at your services,” the young man is quoted as telling the informer, who had presented himself as an Egyptian with a military background.

That moment, recorded on tape and submitted in federal court this week in Camden, N.J., as the authorities charged six Muslim men in the plot, captures something of the complexity of using informers in terror investigations. The informer, sent to penetrate a loose group of men who liked to talk about jihad and fire guns in the woods, had come to be seen by the suspects as the person who might actually show them how an act of terror could be carried off.

Indeed, over the months that followed, as the targets of the investigation spoke with a sometimes unfocused zeal about waging holy war, the informer, one of two used in the investigation, would tell them that he could get them the sophisticated weapons they wanted. He would accompany them on surveillance missions to military installations, debating the risks, and when the men looked ready to purchase the weapons, it was the informer who seemed to be pushing the idea of buying the deadliest items, startling at least one of the suspects.