It’s possible that it was a bomb, I suppose, although supposedly the traces of bomb residue they found were from an earlier security test.
The initial reports in NY had many people who actually had seen something ascending towards the plane, which doesn’t prove anything in itself and was later explained as fireworks. But the Feds immediately launched teams of people to search the dunes. I read one report, one that was gone literally gone by the later edition of the paper, where they had found a tripod and “launching debris” in the dunes. So it wouldn’t have been a SAM (although that’s what most people thought) but some other kind of missile in that case.
I don’t know. This is all coming from what I remember and who knows what was true in those reports and what was confused and what was suppressed.
It would have been a SAM, just not necessarily a MANPAD. Something like the British Javelin or it's follow on the Starstreak
Modern IR Guided SAMS, like the Stinger, or later Russian models, are not restricted to going after the hot exhaust of the engines, they are "all aspect". The Javelin has a ceiling of 5,500 meters or 18,000 feet. It is not even IR guided, but rather uses an RF link back to the launcher and uses Semiautomatic Command Line of Sight guidance.
The gov’t explained away the bomb residue as due to a pre-flight bomb dog test. But when journalist Peter Lance interviewed the police officer involved his log showed a different aircraft tail number at an adjacent gate was tested by him!