Transportation Security Administration spokeswoman Ann Davis says a passenger looking through her overhead luggage had a small Swiss Army knife and another knife with a 2½ inch blade.
Authorities declined to say if the bag's owner or someone else alerted the crew.
Davis says the plane returned to the gate and was cleared of passengers so a check could be conducted.
The passenger was questioned and released.
The TSA is investigating how the knives made it through security.
United Airlines Flight 93, which crashed in the 9/11 terrorist attacks, originated from Newark.
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