The singer, now known as Yusuf Islam, had recently been placed on a government "no-fly" list after U.S. authorities received information indicating associations with potential terrorists, a government official said Wednesday.
The flight was diverted to Bangor, Maine, on Tuesday afternoon after U.S. officials who checked the passenger list learned the singer was aboard. Federal agents met United Airlines Flight 919 and interviewed Islam.
Airlines have access to watch lists and are supposed to screen passengers to make sure those deemed direct threats to aviation do not board planes. The official said it was unclear why United Airlines personnel allowed Islam to get on the flight in London.
A call to the airline was not immediately returned.
Homeland Security Department spokesman Dennis Murphy said Islam was "interviewed and denied admission to the United States on national security grounds" and was expected to be sent back to London on Wednesday.
A second government official, who also spoke on condition of anonymity, said U.S. authorities think donations from Islam may have ended up helping to fund blind sheik Omar Abdel-Rahman, who was convicted for the 1993 World Trade Center bombing, and Hamas, a Palestinian militant group considered a terrorist organization by the United States and Israel.
In July 2000, Islam was deported hours after arriving in Jerusalem. A local paper reported then that the government claimed he had delivered tens of thousands of dollars to Hamas during a visit in 1988. Islam denied ever knowingly supporting Islamic terrorists.
Islam has made a number of trips to the United States in recent years, including one in May for a charity event and to promote a DVD of his 1976 MajiKat tour. He donated half the royalties from his most recent boxed set to the Sept. 11 Fund to help victims of the attacks.
Islam, who was born Stephen Georgiou, took Cat Stevens as a stage name and had a string of hits in the 1960s and '70s, including "Wild World" and "Morning Has Broken." Last year he released two songs, including a re-recording of his '70s hit "Peace Train," to express his opposition to the U.S.-led war in Iraq.
He abandoned his music career in the late 1970s and changed his name after being persuaded by orthodox Muslim teachers that his lifestyle was forbidden by Islamic law. He later became a teacher and an advocate for his religion, founding a Muslim school in London in 1983.
Islam founded Islamia Primary school in London in 1983. In 1998, it became the first Muslim school in Britain to receive government support, on the same basis as Christian and other sectarian schools.
Islam drew some negative attention in the late 1980s when he supported the Ayatollah Khomeini's death sentence against Salman Rushdie, author of "The Satanic Verses." Recently, though, Islam has criticized terrorist acts, including the Sept. 11 attacks and the school seizure in Beslan, Russia, earlier this month that left more than 300 dead, nearly half of them children.
In a statement on his Web site, he wrote, "Crimes against innocent bystanders taken hostage in any circumstance have no foundation whatsoever in the life of Islam and the model example of Prophet Muhammad, peace be upon him."