Belgian police detained 14 suspected Islamic extremists, and the government said they were plotting a jailbreak to free an al-Qaida prisoner convicted of planning to attack US military personnel. Authorities tightened security, warning of a heightened threat of attacks despite the arrests. Police stepped up patrols at Brussels airport, subway stations and the downtown Christmas market, which draws large crowds of holiday shoppers. "Other acts of violence are not to be excluded," said Prime Minister Guy Verhofstadt. The US Embassy warned Americans that "there is currently a heightened risk of terrorist attack in Brussels," although it said it had no indication of any specific targets.