A Delta Air Lines flight from Barcelona to Boston made an emergency landing in Porto on Sunday after smoke filled the cabin.

Flight DL251, an Airbus A330-300 carrying 296 people, left Barcelona at about 3:15pm local time. The crew declared an emergency and diverted to Francisco Sá Carneiro Airport, landing at about 5:45pm.

Portuguese emergency services met the aircraft. Officials said 14 people needed medical attention and four were taken to hospital with smoke inhalation. All were described as only lightly affected.

Reuters reported slightly different figures, saying eight passengers were treated at the airport and three were taken to hospital.

Delta said the diversion followed a reported mechanical issue and that the aircraft landed safely. The airline is working to accommodate the affected passengers.

The exact cause of the smoke has not been established. Aviation authorities are investigating, and the aircraft remained on the ground for technical checks.

Investigators are expected to examine the aircraft’s systems to find the source of the smoke.

Why it matters: cabin smoke events are treated as emergencies because the source can be unclear and the risk can grow quickly.