Younis, a native of Afghanistan, is charged with communicating threats across state lines. The charge carries a penalty of five years in prison.
Younis, also known as Sundullah Ghilzai, allegedly told the woman that he could place a bomb under a sewer head in Georgetown at rush hour to produce the greatest number of casualties, according to an affidavit. He also said that the third and fifth cars on Metro trains had the most commuters, and pipe bombs with specific types of shrapnel could be placed on those cars without being noticed.
The tipster alerted the FBI on Nov. 28.
Magistrate judge Ivan Davis has ordered Younis to undergo a mental health evaluation.
Prosecutors said Tuesday that the FBI is still trying to determine if Younis had the intent or means to carry out the threat.