What happened
According to officials from the Federal Aviation Administration, a passenger mistakenly picked up a backpack containing a Modular Bomb System (MBS). This device is a specialized piece of equipment used for the purpose of training security personnel.
After the passenger boarded the airplane, the aircraft began its climb. During this flight phase, a flight attendant discovered the contents of the bag and notified the flight crew, stating that there was a bag that appeared to be a bomb. Upon receiving this report, the captain instructed the first officer to verify the flight attendant's observation.
In response to the discovery, the captain requested an emergency return to Seattle and requested that emergency equipment be placed on standby. Once the airplane landed, it was directed to a remote location where the crew performed an emergency evacuation. No injuries were reported during the incident.