Is Iran planning attacks on Americans; why is the US on alert?

US intelligence agencies say Iran has substantially increased efforts to target US personnel abroad, with some lethal plots in advanced stages. A memo from Director of National Intelligence Jay Clayton to Secretary of State Marco Rubio cited threats to US officials and ambassadors, as well as potential targets in Europe. Officials cited by The New York Times said Iranian operatives may already be active in the United States. Washington has put personnel in the Middle East on high alert and reduced its diplomatic presence as tensions with Iran escalate.