Several Houstonians grew concerned after spotting a plane circling in the city on Saturday, the ninth anniversary of the Sept. 11 terrorist attacks.

But officials confirmed that the plane was simply a NASA test aircraft.

The aircraft was flying at about 49,000 feet in a racetrack-like pattern near Hobby Airport, said Federal Aviation Administration spokesman Lynn Lunsford.

No further information on the aircraft was available.

Meanwhile, officials said a passenger on a Las Vegas-to-Phoenix flight allegedly made a threat after the plane was forced to return to the airport because of a mechanical problem.

McCarran International Airport spokeswoman Candice Seeley said U.S. Airways Flight 399 departed Saturday afternoon for Phoenix, but the pilot returned shortly after takeoff.

She said “the passenger made some sort of threat” after the plane landed, but she didn’t know specifics.

Police and the FBI told The Las Vegas Sun newspaper that they’re investigating but no arrests have been made.

"It does not appear to be terrorist related at this point," FBI Special Agent Joseph Dickey told the Sun.

On Sept. 11, 2001, terrorists hijacked four commercial airliners and intentionally crashed two of them into the World Trade Center towers in New York City. A third airliner crashed into the Pentagon, and a fourth slammed into a field in rural Pennsylvania. Nearly 3,000 people were killed.

The Associated Press contributed to this report.

anita.hassan@chron.com