In January 2009, two trucks packed with explosives careened toward the front gate of a remote United States Army base in the Khost Province of Afghanistan. The first truck exploded near the gate, injuring a pregnant Afghan woman and several others. But the terrorist plot to kill American soldiers was foiled when the second truck crashed into the blast crater left by the initial explosion.

On Friday, a Texas-born man, Muhanad Mahmoud Al Farekh, was convicted of having helped to plan the attack as an operative of Al Qaeda. After a weeklong trial in Federal District Court in Brooklyn, Mr. Farekh was found guilty on charges of providing material aid to terrorists.

Born in Houston and raised in Dubai, Mr. Farekh, 31, served in Al Qaeda’s external operations unit from 2007 to 2014, prosecutors said, where his duties included collecting money for the terrorist group’s fighters. When he was first identified as a Qaeda operative, his case prompted a debate within the U.S. government over whether it was morally and legally defensible to kill an American citizen overseas without a trial.

Although the Pentagon nominated Mr. Farekh to be placed on the so-called kill list of terrorism suspects and the Central Intelligence Agency pushed for him to be killed, he was taken into custody in Pakistan in 2014 based on intelligence provided by American officials. After being questioned by a team of elite terrorism investigators, he was eventually brought to Brooklyn to stand trial.