The fibers found were ACRYLIC - extremely common.

http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm

It is not possible to say positively that a fiber originated from a particular fabric ….

In order to say that the fiber originated from the item of clothing, the clothing either had to be the only fabric of its type ever produced or still remaining on earth, or the transfer of fibers was directly observed. Since neither of these situations is likely to occur or be known, fiber examiners will conclude that the fibers could have originated from the clothing or that the fibers are consistent with originating from the clothing.

It is argued that the large volume of fabric produced reduces the significance of any fiber association discovered in a criminal case. It can never be stated with certainty that a fiber originated from a particular garment because other garments were likely produced using the same fiber type and color.