Updated at 12:28 p.m. LONDON The British government has warned airlines around the world not to allow Edward Snowden, who leaked information on top-secret U.S. government surveillance programs, to fly to the United Kingdom. A travel alert, dated Monday on a Home Office letterhead, said carriers should deny Snowden boarding because "the individual is highly likely to be refused entry to the U.K." The Associated Press saw a photograph of the document taken Friday at a Thai airport. In London, Home Office officials refused Friday to discuss the travel alert. The diplomat said such alerts are issued to carriers that fly into the U.K., and if any airline brings Snowden into the country, it will be liable to be fined $3,100. The official spoke on condition of anonymity because he wasn't authorized to discuss the matter publicly. The U.K. Border Agency, which operates under the Home Office, has wide leeway to deny entry to people trying to enter the United Kingdom. It has been used to keep radical preachers and extreme right-wing politicians out of Britain. Snowden, an American citizen, has yet to be publicly charged with any crime and no known warrants have been issued for his arrest. U.S. Attorney General Eric Holder told reporters in Dublin on Friday that the case was still being investigated, but said he is "confident that the person who is responsible will be held accountable." Experts believe Snowden's travel options are narrow because of the intense publicity generated by the case and the wide circulation of his photo, which is also contained on the carrier alert. "He's cornered," said Magnus Ranstorp, a research director at the Swedish National Defense College. Even without the U.K. alert, his name is now part of the intelligence matrix and his name would be flagged if he tried to travel anywhere in the world. He can't get into mainland China, they have a very sophisticated database and sneaking in is not easy, and if he tries to fly he won't even get out of Hong Kong airport. He said Snowden might try to use a false passport for travel, and also try to alter his facial appearance by shaving his head and his beard and wearing contact lenses, but would likely be caught. The best option, Ranstorp said, may be for Snowden to follow Wikileaks founder Julian Assange's example and seek refuge in the consulate of a sympathetic country with a diplomatic presence in Hong Kong. Assange, who has spent almost a year inside the Ecuadorean Embassy in London, said he thought Britain had issued the alert on Snowden because "it doesn't want to end up with another Julian Assange." Assange sought shelter in the embassy after Britain agreed to send him to Sweden for questioning about alleged sexual assaults. Assange argues that could lead to extradition to the U.S., where he and WikiLeaks are being investigated. "The United Kingdom doesn't want to say no to the United States under any circumstances," Assange said. Not in my case and not in the case of Mr. Snowden. He said Britain should be offering Snowden asylum, not excluding him. It isn't clear if other Western European countries have also alerted airlines not to bring Snowden into their countries. Officials in France, Germany, Spain and the Netherlands said they have not issued any warnings about possible travel by Snowden. In Amsterdam, foreign ministry spokesman Weibe Alkema said the government would execute an arrest warrant for Snowden if one had been issued. At one point, Snowden expressed an interest in seeking refuge in Iceland, but the government there said no contact had been made. In London, some human rights activists expressed disappointment with the British government's stance. Martyn Day, from the law firm Leigh Day, said it was "depressing" that Britain's approach is so tied to the U.S. "Mr. Snowden has been entirely open about being the whistle-blower on what is alleged to be one of the most outrageous invasions of privacy the world has ever seen," Day said. If he wants to come to the U.K., he should be allowed to do so. Even without criminal charges, Snowden's world is now shrinking. If other countries follow Britain's example and bar his entry, Snowden would have few options if he weren't allowed to stay in his preferred sanctuary of Hong Kong, a semiautonomous Chinese territory. The document titled "RALON Carrier Alert 15/13" has a photograph of Snowden and gave his date of birth and U.S. passport number. It said: "If this individual attempts to travel to the U.K.: Carriers should deny boarding." "Carrier alerts" are issued when the U.K. government wants to deny entry to people who don't normally need visas to enter the country, as is the case with most U.S. citizens, or already have visas but something has happened since they were issued, the diplomat said. People who are turned away are told in writing why they have been rejected, and some - but not all - have the right to appeal the ruling. A Bangkok Airways officer said the airline was notified on Thursday about the alert by the Airports of Thailand PCL, which operates national airports throughout the country. She said the notice wasn't intended to be seen by the public. The officer spoke on the condition of anonymity because she wasn't authorized to give the information to the media.