The United States has told its diplomats overseas to identify groups whose members should be thoroughly investigated when they request a visa. The State Department sent a series of directives to U.S. embassies and diplomatic offices earlier this month. The department told them to investigate the social media messages and activities of visa applicants. It said officials should be looking for people who are suspected of terrorist ties or of having been in areas controlled by the Islamic State group. Another directive ordered embassies to set up security and intelligence working groups to establish guidance for “population sets.” It said these measures would identify which people require a detailed investigation before they are permitted in the United States. Even if someone is qualified for a visa, they could still be barred from entering the country if they do not meet the rules set by the working groups. The directives are the first evidence of a Trump administration plan for the “extreme vetting” of foreigners before they are given visas. Before becoming president, Donald Trump promised such a plan to American voters during the 2016 election campaign.