BBC West investigations uncover reports expressing serious concerns about parts of a site where four people died.
The Health and Safety Executive will continue investigating the explosion at Avonmouth in 2020.
Luke Wheaton, 16, and three men were killed in the explosion at a water recycling centre in Bristol.
The three men and a teenage boy died in the explosion at the Wessex Water site last week.
The teenager and three men were killed in the blast at a water treatment works in Bristol.
Three employees and a contractor were killed in the blast at a water recycling centre in Avonmouth.
Another person was hurt when a "very loud explosion" was heard from a silo at the site in Bristol.
An investigation is under way into a fatal blast at a waste water treatment works in Avonmouth, Bristol.
Eyewitness Patrick McCrory spoke to one of the Wessex Water workers after the explosion.
