LONDON — The British government said Monday that it rejected an application by the Nigerian man accused in the failed Detroit airliner bombing plot for a renewed student visa in May, and that he was placed on an official watch list to prevent him from re-entering Britain.

In a BBC radio interview, Home Secretary Alan Johnson said he did not believe that the suspect, Umar Farouk Abdulmutallab, had acted alone in a plot investigators say could have brought down a Northwest Airlines flight as it approached Detroit with more than 280 people aboard on Christmas Day.

Mr. Johnson gave no reason for suggesting that Mr. Abdulmutallab might have had accomplices. But he noted that Scotland Yard and Britain’s security services were investigating whether the suspect’s Islamic beliefs were radicalized while he was a mechanical engineering student from 2005 to 2008 at University College London, one of Britain’s elite academic institutions.

Mr. Johnson said Mr. Abdulmutallab’s application to renew his student visa was rejected in May after officials had determined that the academic course he gave as his reason for returning to Britain was fake. The secretary said the suspect was then placed on the watch list, a procedure that would normally involve informing American authorities of the action Britain had taken.

But Mr. Johnson was vague about whether officials in the United States had officially been informed of the action, although he said he doubted that there had been a “hiccup” in the process.

His vagueness appeared to be part of a wider uncertainty on the part of British authorities as they scrambled to learn more about Mr. Abdulmutallab’s activities in Britain and about his associates while he was studying here.

The rejection of the visa renewal appeared to have been part of a wider process initiated by British authorities this year when they began to crack down on so-called fake colleges that officials said had been established in large numbers across Britain in an attempt to elude tightened immigration controls. Saying the colleges had been used to obtain thousands of fraudulent student visas, the Home Office has closed about 2,000 of them in the past six months. University College London was not part of the crackdown.

The fact that Mr. Abdulmutallab had been barred from entering Britain raised new questions about one of the most pressing issues stemming from the bombing attempt — the efficacy of systems in place on both sides of the Atlantic for guarding against terrorists planning attacks on airliners.

Officials in the United States have said Mr. Abdulmutallab was placed on a “wide” American watch list of about half a million names after his father contacted the United States Embassy in Nigeria in the fall to say that he was concerned about his son’s activities.

But Mr. Abdulmutallab was not on a much narrower “no-fly” list that might have prevented him from boarding the Detroit flight in Amsterdam. Mr. Johnson, the home secretary, left open the possibility that Mr. Abdulmutallab might have passed through British airports on his travels after his visa application was rejected, a possibility he said was routine.

The issues being investigated by the police and the security services included “what happened when he was in this country, was he radicalized in this country, was there any association with whoever may have been behind this plot,” Mr. Johnson said. “We don’t know yet whether it was a single-handed plot,” or whether other people were involved. “I suspect it’s the latter rather than the former.”

On Monday, the police were continuing their search of apartments in a mansion block near the University College London campus where Mr. Abdulmutallab lived in a family apartment during his engineering studies.