The new details emerged Saturday, a day after the deaths were announced in a brief statement.
The official, who spoke on condition of anonymity because he wasn't authorized to release the information, says the attack occurred in Wardak province.
First reports indicated three Americans were killed but the official said one of the dead was an Afghan.
It wasn't immediately clear why the interpreter opened fire.
A clash between U.S. and Afghan troops Saturday that left four Afghan soldiers dead also occurred in Wardak province. But the official says the incidents don't appear to be related.
By Associated Press Writer Kim Gamel