WASHINGTON, (Reuters) – The United States is  investigating whether al Qaeda was involved in a Christmas Day  attempt to blow up a passenger jet, but there is no early  evidence the Nigerian suspect in the case was part of a larger  plot, the U.S. homeland security chief said yesterday.

U.S. authorities on Saturday charged Umar Farouk  Abdulmutallab, 23, with attempting to blow up a Northwest  Airlines plane as it approached Detroit on a flight from  Amsterdam with almost 300 people on board.

Asked whether al Qaeda was involved, Homeland Security  Secretary Janet Napolitano told ABC’s “This Week” program,  “That is now the subject of investigation, and it would be  inappropriate for me to say and inappropriate to speculate.”

A senior U.S. official, speaking on condition of anonymity,  said on Saturday authorities were looking at the possibility  that Abdulmutallab had ties to al Qaeda in Yemen.

“Right now, we have no indication that it is part of  anything larger,” Napolitano told CNN’s “State of the Union.”

Yesterday, the crew of that same Amsterdam to Detroit  flight reported an emergency incident because of an unruly  passenger. The plane landed safely and a Nigerian man was taken  into FBI custody. The man had raised concerns because he spent an unusually  long time in the plane’s bathroom, but it turned out he had a  “legitimate illness,” the Department of Homeland Security  officials said.

President Barack Obama, on vacation in Hawaii, stressed the  importance of maintaining heightened security for air travel,  the White House said after yesterday’s short-lived scare.