Tell No One by Harlan Coben

A Doctor and his wife, who has been his love since they were five years old, are on a trip to the family’s private lake when they are attacked.  He was swimming and could not get to his wife fast enough, and when he did reach the dock, he was knocked back into the water.  They found the body a few days later.  She had been stabbed repeatedly and branded with a ‘K’ just as several other victims of a serial killer.

Now, nine years later, he gets a mysterious email that has a feed from a security camera and his wife looks at him for 30 seconds.

He starts asking questions, digging up the old case and trying to find out what is going on.  This brings the attention of the FBI, who think he was the real killer.  More and more damning evidence turns up and he has to run to keep ahead of the authorities while he searches for the truth.