Bomb threats affected several flights on Tuesday, including a a US Airways flight from San Diego to Philadelphia.

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NEW: Delta, United, US Airways and Volaris had flights targeted by bomb threats Tuesday

It is the 2nd time in 8 days that bomb threats have disrupted air traffic

CNN  — 

A string of bomb threats disrupted air traffic Tuesday, and while a law enforcement official says they do not appear credible, it marks the second time in eight days that authorities had to investigate numerous threats against airlines.

At least four commercial flights were among Tuesday’s targets, according to airline representatives.

Law enforcement officials met United Airlines flight 955 after it arrived at Chicago’s O’Hare International Airport, clearing the flight, an airline spokesperson said.

“Passengers exited the plane as usual. We are working with law enforcement in their review,” the spokesperson said.

Delta flight 55 from Lagos, Nigeria, to Atlanta landed routinely Tuesday morning “after an evaluation … in conjunction with authorities, deemed the threat non-credible,” a Delta spokesperson said. The 13-hour, 5,800-mile flight landed ahead of schedule.

Law enforcement met US Airways flight 648 from San Diego to Philadelphia in Philly “because of a possible security threat to the aircraft,” an airline spokesperson said.

The A320 was carrying 88 passengers and five crew, who landed safely and were transported to a terminal building where a customer service representative assisted them.

Authorities also searched Volaris Flight 939 from Portland, Oregon, after it landed in Guadalajara, Mexico, on Tuesday “as a precautionary measure due to a phone threat,” the airline said in a statement.

Last Tuesday, NORAD scrambled two F-15 fighter jets over the Atlantic to escort Air France flight 22 to New York’s John F. Kennedy airport after Maryland State Police reported getting a threat about the Paris-to-New York flight.

At least 10 threats were phoned in about flights that day.

CNN’s Rene Marsh, Pamela Brown, Fidel Gutierrez, Mayra Cuevas and Simon Hernandez-Arthur contributed to this report.