(CNN) -- A Virginia man has been accused of threatening to place pipe bombs along the Metro transportation system in Washington. According to a federal complaint, Awais Younis, who was arrested December 7, related his plans to someone on the social networking site Facebook. Younis, also identified as Sundullah Ghilzai, told an FBI complainant through Facebook how how to build a pipe bomb "and what type of shrapnel would cause the greatest amount of damage," according to federal authorities.