Hidden witness

the long version

"When Detective Superintendent Henry Christie takes on the case of a brutal murder of a seemingly innocent old man, the investigation brings him into touch with old friend and FBI agent Karl Donaldson, who is trying to track down a hit man believed to have entered the UK illegally. It's obvious the two cases are connected, but it soon becomes clear there's more to the old man's murder than simple underworld retribution"--Publisher's description.