The woman who had the carry-on bag was questioned by authorities, along with her sister, and both women were later released.
Authorities say they are trying to determine Lolita Austria, 57, of Rochester, N.Y., wound up with the bag containing the phony grenade.
Police say one possibility is that she might have accidentally picked up the bag at the security checkpoint.
The San Diego Unified Port District Harbor Police, in a statement, said that "the checkpoint security personnel had successfully completed a test using the test grenade on the same machine, moments before Ms. Austria entered" the security checkpoint area.
"She made a big mistake, one way or the other," said Port District spokeswoman Rita Vandergaw. "Whether it was inadvertent or not, she shouldn't have had that bag."
The pilot of Flight 788 stopped the MD-80, bound for Chicago, on the taxiway and authorities removed the grenade. Before the plane's delayed departure, all passengers were taken off and re-screened.
Police say the phony grenade was apparently discovered by Austria, who found it when she pulled a shirt out of the bag and then "began to panic," telling flight attendants that the grenade did not belong to her.
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