Federal and airline officials are still trying to figure out what crumpled the nose cone of a Northwest Airlines Boeing 757 that landed safely Sunday in Tampa, Fla.

One early theory has been ruled out: that the plane struck a bird.

"At 18,000 feet, a bird would have to be wearing an oxygen mask," said Federal Aviation Administration spokeswoman Elizabeth Isham Cory in Chicago. Other possible causes include metal fatigue or a design failure, she said.

The pilots reported hearing a bang and losing the radar in the aircraft's nose when the Detroit-to-Tampa flight was approaching Tampa's airport at 18,000 feet. But the cause remains unknown, and officials said the investigation could last several weeks.

"When something like this happens, you always look at how it might affect the entire fleet of similar aircraft," Cory said. "But at this point it looks as if it's a very isolated incident."

The flight was in no danger as a result of losing radar because backup information was available from ground controllers, Cory said. The landing was not considered an emergency.

Northwest removed the nose cone from the plane and returned it to the Twin Cities on Monday night for further analysis, Northwest spokeswoman Kristin Baur said. The aircraft was repaired overnight in Tampa and returned to service on Monday, she said.