WASHINGTON — US federal authorities have arrested an Afghan man for  threatening to blow up Washington's subway system on Facebook, the  Justice Department said Tuesday.
Awais Younis, also known as  Sundullah "Sunny" Ghilzai and Mohannme Khan, allegedly described how to  build a pipe bomb that could maximize casualties on the Washington  Metrorail, according to a December 6 criminal affidavit.
During a  chat last month Younis said the third and fifth cars on the Metro had  the highest number of commuters on them and that he could place bombs  there without being noticed, according to the affidavit.
"Complainant  responded by saying 'You wouldn't do that,' and (Younis) responded by  saying 'Watch me,'" the affidavit says, without providing further  details on who turned Younis in.
Younis had also allegedly posted  pictures from Afghanistan on Facebook in which he holds an AK-47 assault  rifle and his uncle stands in front of a tent filled with explosives,  with a caption reading "My family business."
He had also recently posted a message saying: "Christmas trees were going to go boom."
Younis threatens the complainant in a chat included in the affidavit.
"You are sticking your nose where it doesn't belong into something bigger then you and I," he said.
"That  is the problem with Americans they can't leave well enough alone until  something happens then they sit there wondering why we dropped the twin  towers like a bad habit hahaha."
He goes on to warn the complainant to tell his father not to take the Metro to work.
Younis,  who will appear in court on December 21, has been charged with  "communicating threats using interstate communications" but not  terrorism, the Justice Department said in a statement.
"The public  should be reassured that his activities prior to his arrest were  carefully monitored and that there is no threat against Metrorail or the  general public in the Washington, DC area," spokesman Dean Boyd said.
An FBI spokeswoman said Younis was in his early 20s.
News  of the arrest comes less than a week after FBI agents used Facebook to  nab a young Muslim-American who believed he was about to set off a car  bomb at a US military recruitment office north of Washington.
In  October a Pakistani-American man was arrested for plotting a series of  bomb attacks on Washington's subway system with people he believed were  tied to Al-Qaeda
Farooque Ahmed, 34, had been allegedly observing,  videotaping and photographing Metro stations in the Virginia suburbs of  Washington, including near the Pentagon and Arlington National  Cemetery.
AFP