SAN DIEGO — A United Airlines plane returned to San Diego International Airport Tuesday morning shortly after take-off when an external laptop battery pack caught fire in the cabin.

United Airlines Flight 2664, a Boeing 737 on route to Newark Liberty International Airport in New Jersey, landed safely back in San Diego around 7:30 a.m., Federal Aviation Administration officials said.

Emergency crews responded shortly after when the plane was back on the ground, with the responders bringing the aircraft safely from the runway to Gate 43.

The device was placed in a fire bag by the airplane crew to prevent the blaze from spreading, according to the San Diego Fire Department.

Four passengers were transported to a UCSD Health hospital with minor, non-life-threatening injuries due to smoke inhalation. Two customers on the flight opted not to be transported.

United Airlines in a statement to FOX 5 said that those transported to the hospital were flight attendants and were taken there as a precaution.

A second plane was brought in for the passengers on the flight, which took off for New Jersey around 11:45 a.m.

San Diego International Airport put out an alert 2 notice in response to the incident this morning, which is designated for when an aircraft is having major difficulty approaching an airport.

“This morning (Tuesday) we did have an alert 2 that was responded to once they were able to land at the airport. That then makes all of the emergency crews come out and we were able to get the plane from the runway to the gate safely,” said Sabrina LoPicolo, a spokesperson for San Diego International Airport.

There is no information yet on what caused the battery pack to catch fire. The FAA will be investigating the incident.

United Airlines sent this statement in regard to the incident:

“United flight 2664 safely returned to San Diego after a customer’s battery pack ignited. Our crew acted quickly to contain the device and medical personnel met the aircraft upon arrival at the gate. Several flight attendants were taken to the hospital as a precaution and two customers were evaluated onsite. 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.”