Mobile phone records show that Jessica's phone sent its final "disengage" signal at 1846 BST when it disconnected from the network.
Most of Soham is served by a mobile phone mast in the north of the village but there are a few "hotspots" where phones link to the mast at Burwell, to the south.
Ian Huntley's house is one such hotspot and the disconnection signal from Jessica's phone was received at Burwell.
Huntley said that the girls died in his bathroom where he was helping Holly with a nosebleed. He said he accidentally knocked her into the bath and then smothered Jessica because she was screaming.
But a pathologist said the bathroom was too small for Holly to have fallen into the bath unless someone had lifted her legs over the side.