Why do the NE Patriots have the number 91 on their helmets?

The 2007 Patriots are wearing circular stickers on the backs of their helmets with the number 91 on them, in honor of their teammate, the late Marquise Hill.

Hill was a defensive end with the Patriots. He died in a jet skiing accident in May of 2007. Hill, who is from New Orleans, had devoted himself to helping victims of Hurricane Katrina. He was 24 when he died. He was the first Patriots player to die while on the active roster.