Gun planted by staff at Logan Airport perhaps?That would be consistent with the PROVEN accounts of plans to hijack at least one more plane, and the PROVEN accounts of security sweeps finding (at least) boxcutters on more planes.
JFK is at least 10 times worse than Logan. So it is believable there was other on-the-ground assistance, too. It also makes the reasons for choosing JFK stronger - easier to corrupt or infiltrate.
AND it would be VERY inconvenient for ailines to have to really implement REAL security quickly. Lots of motive for coverup.