Higgins was looking at videos coming out of the Daraya region when he noticed several weapons that he had not yet documented in Syria, among them a portable anti-tank rocket and a wheel-mounted recoilless rifle.

Source- New Yorker https://www.newyorker.com/magazine/2013/11/25/rocket-man-2

It seems to me that the entire phrase is modifying weapons.

So the phrase has a noun and Among them --prepositional phrase . what does that make it?