As investigations continue into the bombs shipped from Yemen to the United States, the news last week about a plot to bomb Washington, DC Metro stations should not be forgotten. Indeed, the arrest of Farooque Ahmed, a naturalized U.S. citizen born in Pakistan, is evidence of another kind of ongoing terrorist threat to the United States, one that stems from citizens and immigrants. Ahmed’s arrest shows the threat of smaller-scale terrorist attacks carried out by U.S. citizens is growing. U.S. citizens are particularly attractive to al Qaeda and other terrorists because they can more easily plot and prepare for attacks without drawing the attention of law enforcement, the intelligence community or the Department of Homeland Security.