Phoenix police and FBI bomb technicians searched the plane, originally bound for Austin, Texas, and found no explosives, the FBI said in a statement.
All passengers have been interviewed and are in the process of boarding another aircraft to Austin, the airline said. Their luggage had been returned to them after TSA agents re-screened all the bags.
Authorities are still trying to determine who called in the threat, which officials say was specific to that flight.
"All efforts are being made to identify the caller who initiated the bomb threat," said FBI Special Agent Manuel Johnson of the agency's Phoenix office.
Southwest flight 2675 was midair over Arizona when it made a 180-degree turn and began its rapid, unscheduled descent into Phoenix.
The airline said the captain rerouted the plane "out of an abundance of caution."
Edward Burger was on his way home to Austin after giving a lecture at UC Berkeley when the pilot announced that there was a security threat. After making the turn, Burger said, the plane was escorted by two fighter jets until it landed at Phoenix Sky Harbor International Airport.
The aircraft made a “very smooth landing and then taxied to a distant area with FBI, police and bomb trucks,” Burger said via email.
Passengers exited the plane in a very orderly manner, he added, and were told to leave all of their luggage and belongings behind. Buses were waiting to take the passengers to a secured area, said Sgt. Steve Martos of the Phoenix Police Department.
While quarantined in an airport terminal, Burger and other passengers were told not to use their cellphones.
“Each passenger was interviewed and asked four questions,” he wrote. They then had to wait to retrieve their belongings and board the new plane to Austin.
At the latest status check, the flight was expected to arrive in Austin at 2:30 a.m., nearly eight hours later than the original arrival time.