BEIJING -- Eight baby mice were found aboard a United Airlines flight from Washington but tests showed they were free of dangerous viruses, the government said Thursday.

The mice were found by a cleaning crew in Beijing after the flight arrived Sunday, according to the airline.

The mice were tested for hantavirus and other dangerous viruses and results were negative, China's Administration for Quality Supervision, Inspection and Quarantine said on its Web site.

The mice were found in a pillowcase in a bin for unused pillows, said Robin Urbanski, a spokeswoman for Chicago-based United. She said no passengers were on board at the time.

"Our customers on this flight were always safe and unaware of the situation," Urbanski said.

The airline told Chinese authorities about the mice, which is standard procedure, and officials "thoroughly inspected (the plane) for a little over two days," she said.

The plane was back in service and flew to San Francisco from Beijing on Wednesday, Urbanski said.

Chinese authorities did not impose any sanctions or fines on the airline, Urbanski said.