The kidnapper cuts off a lock of the victim's hair and sends it to the parents. The police look for the victim's hair on her toothbrush to compare DNA for determining whether the sample is genuinely from the victim. Until recently this would not have worked: hair needed to have the follicle (root) attached for DNA to be found. However a forensic breakthrough in 2019 now permits some degree of DNA comparison between cut hair and a known sample, though at a much greater cost.
At 32:53 min. the camera man is seen behind Professor T. in a reflection of an outside window.