The Velociraptor Holding Pen was located some distance behind the Visitor’s Center on Isla Nublar. The Holding Pens were surrounded by a twelve foot high double barrier chain-link fence topped with spirals of barbed wire. It was electrified, as a precaution due to the previous escapes of the Velociraptors.In addition, the Holding pen was outfitted with electronic sensors to warn of another impending escape. The Holding Pen housed eight adult females, while the staff figured out how to properly integrate them into the park setting.
When Grant, Malcolm and Ellie went to view the Velociraptors in their containment, the raptors attempted to ambush them. However due to the electrified fence, the attack failed. However, it confirmed Malcolm’s theories that the raptors had escaped before. At 5:14:57 am, the auxiliary power was put into place, but due to the power not being sufficient to operate the fences, the remaining raptors escaped.
However, some must have escaped before this, due to the official animal count of 37 before the main power is turned off. Additionally, Tim Murphy spots an escaped raptor in the Hypsilophodon paddock hours before the power is turned off. Due to the reaction by one of the workmen after talking with Malcolm, the Velociraptors frequently attacked the fence, whenever they saw an opportunity.
Jurassic Park by Michael Crichton page 114,116,147,152,164,302 (First Edition, Thirty Fourth printing)