(SAN DIEGO) -- Four people were taken to the hospital after an external battery caught fire on a United Airlines flight early Tuesday, officials said.
Shortly after takeoff, the device was found on fire inside a seat back pocket of first class, United told ABC News. Officials said the flight crew prevented the fire from spreading by placing the battery into a thermal containment bag.
"Our crew acted quickly to contain the device," the airline said in a statement.
The Newark-bound flight then safely returned to San Diego International Airport where it was met by the local fire department and medical personnel.
Four flight attendants were taken to the hospital as a precaution and two passengers were evaluated on site, according to United.
"We thank our crew for their quick actions in prioritizing the safety of everyone on board the aircraft and we are making arrangements to get our customers to their destinations," United said.
Last year there were 57 incidents involving lithium batteries on planes, according to the Federal Aviation Administration.
The FAA said it will investigate the incident.
ABC News' Matt Fuhrman contributed to this report.