The Severn Tunnel, which connects south Wales with London and Bristol, is closing for 16 days.
A man and woman, both from Chepstow, were arrested on suspicion of perverting the course of justice.
The nine-month-old suffered a head injury consistent with a dog bite at his father's home, an inquest hears.
The nine-month-old baby boy died at the scene in south-east Wales on Sunday evening.
Gwent Police say a baby boy died following an attack in Monmouthshire.
Police and paramedics attended a house in Rogiet, Monmouthshire, on Sunday evening and a dog was removed.
