The police said on Sunday that a knife had been recovered and posted a picture on social media. The next day, however, it posted another message saying the knife had been taken from the crime scene by an unidentified man.

Officers recovered a different knife from the scene, thinking it was the one that the suspect had been carrying, according to the Gothamist.

An NYPD spokesman told the news website that the knife they picked up must have been left behind by another subway rider.

https://www.bbc.com/news/articles/c93y74xl1wvo

So they just accidentally recovered some unrelated knife, thinking it was the suspect's knife, while failing to recover his actual knife that he charged them with. I don't know what the truth is, but that story is genuinely ridiculous.