The European Union's head office estimates that the cost of fully restoring border controls between EU member states would be as high as 18 billion euros ($20 billion) a year. In a planning document on how to fully restore the Schengen zone by December at the latest, the Commission said that beyond trade, the re-imposition of borders "would also risk putting in jeopardy the judicial and police cooperation." Swedish police say a male asylum-seeker has been stabbed to death at a refugee center in central Sweden, adding three suspects, also asylum-seekers, have been detained. Police spokesman Stefan Wickberg says they had no immediate motive for Friday's pre-dawn stabbing at an asylum center north of Lindesberg about 200 kilometers (124 miles) west of Stockholm. Since the start of the year, at least three people have been murdered in asylum centers in Sweden, which has accepted the highest number of migrants per capita in Europe. In Athens, Prime Minister Alexis Tsipras was briefing opposition leaders on the immigration crisis that has left more than 32,000 people stranded in Greece. The United Nations refugee agency has criticized European Union leaders for warning migrants not to come to Europe when most people arriving are from conflict zones rather than looking for work. France's top security official says the population of the sprawling Calais migrant camp is now at 3,800 people, down from a peak last year of 6,000, after and the inhabitants were relocated and many of the shanties later dismantled. Interior Minister Bernard Cazeneuve told BFM television on Friday that France has set up 102 shelters across the country for migrants, and blamed a handful of extremists for inciting them to protests that have included some who stitched their lips together.