(NECN/ABC) - Concerns about international Terrorism increased this week, as Great Britain raised its terror threat level to "severe." In the United States, intelligence officials are again tightening their focus on airline passengers, this time to include potential female terrorists. Airline security officials were on the lookout for possible female suicide bombers, traveling with Western passports on U.S.-bound flights. U.S. intelligence officials issued the alert, believing the women may have been trained by al-Qaeda in Yemen. "al-Qaeda is looking to put together an attack using people that would not fit a profile that we'd be looking for," ABC News consultant Dick Clarke said. Authorities told ABC News they received the information from accused Northwest Airlines bomber Umar Abdulmutallab. He provided the FBI with specific names and descriptions of other who trained with him in Yemen. U.S. officials told ABC News that last weekend six different people on the No-Fly List were stopped from boarding U.S. flights. Two of them were stopped at London's Heathrow Airport, attempting to board an American Airlines flight to Miami and a United Airlines flight to Chicago. The four others were stopped while attempting to board flight in Nairobi, Kenya, the Caribbean island of Saint Maarten, Fort Lauderdale and Minneapolis. None was arrested or detained, although one was sent back to Saudi Arabia. Some law enforcement officials believe the six individuals who were stopped appear to be part of a well-organized probe to find weak spots in the airline security system. This news comes as the United Kingdom raised its terror alert level on Friday. "The fact that we've moved to another threat level means that we've put more resources in, we've heightened the state of vigilance, it shouldn't be thought to be linked to Detroit or anywhere else for that matter," Alan Johnson said. ABC's David Kerley reports.