Authorities are trying to find out who made a threat that forced a New York City-bound Air China flight to turn around and return to Beijing in the middle of the trip.

Information about the threat, the nature of which is still unclear, was provided by the US, but "it could have been forged and released from inside the country", a Beijing airport police spokesman told China Daily on condition of anonymity.

Officials are making efforts to track down who sent the false threat, he said.

Flight CA981 was scheduled to take off from Beijing Capital International Airport at 1 pm on Wednesday and land 13.5 hours later at New York City's John F Kennedy International Airport.

After taking off at 1:30 pm, the Boeing 747 jumbo jet, which had more than 300 passengers, returned to the Beijing airport at 8:25 pm on Wednesday.

"We were told there were unsafe factors on that flight," an Air China spokesman said, declining to elaborate.

Other sources in China, including the country's civil aviation authority and airport police, also declined on Thursday to reveal specifics of the threat.

All passengers, their carry-on and checked-in luggage, and cargo on board were re-screened. Police also searched the plane's passenger and cargo cabins. Nothing suspicious was found, airport police said.

"Flight safety is too important. We won't take any risks," Yang Rui, deputy general manager of Air China North America, told China Daily.

The airline later changed the plane and the cabin crew. The flight took off at about 12:30 am on Thursday.

"Some passengers opted to abandon their trips, but most of the passengers have boarded the flight," he said.

According to the airline's website, the flight arrived in New York at 12:38 am on Thursday local time.

Air China, the country's flagship airline, first disclosed the incident through its Sina micro blog on Wednesday night. Some netizens praised the airline for its transparency.

One passenger - Wang Qiang, an official in the Ministry of Science and Technology - said the incident was handled very smoothly.

"The airport and police did a great job. All passengers cooperated and didn't cause any trouble, and we strongly supported the investigation," Wang wrote on his micro blog shortly before the flight's second departure from Beijing.

Wang first thought something was wrong when the on-board electronic flight map showed the plane heading back to Beijing. However, flight attendants told him it was a map display error.

The company later explained the crew members did so to avoid unnecessary panic.

The Air China spokesman denied rampant speculation on social-media sites that the flight returned because a wanted corrupt official who was trying to flee the country was on board.

The spokesman added that the incident is not the first time the airline has received fake threats.

On Aug 8, an Air China flight from Beijing to Nanchang returned after a passenger on board claimed there was a bomb on the plane. It turned out to be untrue.

In April, an Air China flight from Nagoya, Japan, to Chengdu via Shanghai also had passengers and luggage re-screened when it made a stop at Shanghai Pudong International Airport. The airport said it received an anonymous call claiming there was a bomb on the plane, which turned out to be false.

Other airlines have also encountered such situations. A New York City-bound Japanese flight returned in the middle of the journey on July 31 after it received an e-mail that said a bomb was on the flight. Police said they did not find one.

Contact the writers at xindingding@chinadaily.com.cn and yuwei12@chinadailyusa.com.

Wang Jun in Los Angeles contributed to this story.