A United Airlines ual flight from Newark, New Jersey, to San Diego was diverted to Denver on Wednesday because of an unspecified security concern, the airline said.

United Flight 1243 landed safely at about 9:30 p.m. local time at Denver International Airport, where passengers got off the plane, an airline spokeswoman said.

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The airline said the flight was diverted to Denver in an “abundance of caution” because of an unspecified security concern.

Passengers boarded the airplane to resume their flight to San Diego after a two-hour delay, United said.

Denver police and the FBI were investigating the incident, the airline said.