The storm drain complex Noah Donohoe entered before he drowned was inspected the previous week but an access hatch remained unlocked, according to a letter seen by BBC News NI.
The padlock on the metal drain cover in north Belfast was only added after the teenager disappeared.
One expert said the hatch should have been locked at all times.
A letter written by the Infrastructure Minister Nicola Mallon revealed the inspection was carried out on 18 June.
This was three days before Noah entered it - but no padlock was placed on the access hatch.
A padlock was later fitted on the metal grille cover on 24 June.
Noah's body was found in the storm drain on 27 June and a post-mortem examination found he died as a result of drowning.
The Department for Infrastructure (DoI) declined to answer questions about the contents of the letter.
A spokesperson said: "The information that you have requested relates to ongoing police and coroner investigations.
"The department cannot release information into the public domain which would undermine the progress of such investigations."
According to the minister's letter, the lock on the metal grille covering the storm drain at the back of Northwood Road was only fitted after consultation with the police.
Ms Mallon wrote: "The padlock on the hatch within the grille structure…was fitted on the 24th of June, after discussions with the PSNI.
"The hatch was unlocked up until this time."
What we know about Noah's final journey
The police have changed the time of the last confirmed sighting of Noah Donohoe before he disappeared.
This is significant as it means the final part of his journey after he entered the Northwood estate in north Belfast is no longer unaccounted for.
Initially the police said the last sighting of Noah was at 18:11 BST on 21 June.
But a recent inquest hearing was told that police have now revised that time to 18:03 - a difference of eight minutes.
The new time means there is no gap in timings when Noah enters the Northwood estate.
Coroner Joe McCrisken has said it seems likely that Noah entered the drain by lifting the unsecured metal drain cover.
An expert safety engineer, Dr Ciaran McAleenan from Ulster University, said the metal access grille should always have been locked.
"They are designed to be locked, so clearly it could always have been locked.
"In my view it should always have been locked from the moment it was constructed and any safety inspection could have picked that up.
"The moment it was picked up on inspection, a lock should have been put on it."
Residents living close to the drain said they feared a repeat of what happened to Noah and have questioned why access to the storm drain structure was still possible from back gardens and a neighbouring housing estate.
Michael Henry, who lives nearby, said: "We have seen what happened. What's to say it could not happen again?
"I have two young children of my own and if they got out, I would be afraid of them going down to the drain."
Safety engineer Dr McAleenan was surprised the entire storm drain structure was not fenced off to prevent public access.
"The problem with this particular culvert is that it is open to the public from one end," he said.
"If you are looking at designing a structure of this nature, you should be able to close it completely.
"The normal would be to have a palisade fence or something that would totally surround it - so that there's no public access to get to it.
"What we have here is a palisade fence to one end of it - which is locked - but at the opposite end it is totally open.
"I would have this fenced off completely immediately."