According to Wiki:

British singer formerly known as Cat Stevens was denied entry into the United States on 21 September 2004, and would remain banned until December 2006, when he was re-admitted without incident to perform at several radio concerts and to conduct interviews promoting his new album.[8] His name had originally been flagged as being on a no fly list. Customs and Border Protection (CBP) officers alerted the United States Transportation Security Administration, which then diverted his flight to Bangor, Maine, where he was detained by officers from the United States Department of Homeland Security (DHS).[9] The following day, he was flown back to the United Kingdom. A spokesman for the DHS stated that there had been "concerns of ties he may have to potential terrorist-related activities".[10] The singer said that it may have been an error due to the similarity of his name with that of one "Youssef Islam".[11]