Specialist gas investigators have carried out a scene inspection of the Kaitaia College engineering workshop ripped apart by an explosion on Monday.
Occupational Safety and Health called in three experts to find the cause of the blast that left four students and a teacher badly hurt.
Northland OSH manager Ian Baxter says the investigators tried to reconstruct the room to get a clear idea of where the explosion came from. He says the acetylene and oxygen gases used by the students for welding were piped into the workshop from tanks in an annex, but the tanks themselves did not explode.
Baxter says the investigators have taken samples from the scene back to Wellington for analysis.
The condition of the pupils hurt in the explosion has improved, with the most seriously injured now off the critical list.
Two of the 15-year-old students are in Auckland Hospital and two boys and their teacher are in Whangarei Hospital. All have head and facial injuries.
Auckland Hospital says the student who was on life support after surgery is now conscious and has been moved to a ward.
The other boys and the teacher are also in a stable condition.
Following the explosion the Ministry of Education is sending out a safety warning to schools about acetylene gas.
Spokesperson for the ministry, Jenny Bridgen, says there was acetylene gas welding equipment in the workshop and OSH has asked the ministry to send out safety reminders.
