The factory at the centre of a major toxic leak which injured 37 people has been closed down while investigators try to trace the cause.
One man, who suffered chemical burns at BASF's Seal Sands site near Billingham, remains in a serious but stable condition in hospital.
A second person will need plastic surgery, according to Cleveland Police.
The company said it was working closely with the Health and Safety Executive (HSE) and the Environment Agency.
Ambulance and fire crews from across the region responded to the leak, which happened on Thursday morning at the firm's Nylon Intermediates plant.
Fifteen people were treated in hospital and later released.
A further 20 workers were decontaminated at the site and are now back at work, according to the firm.
The leak happened at BASF's Nylon Intermediates plant
The casualties suffered burns, skin irritation and breathing difficulties.
Torben Berlin Jensen, BASF's managing director, said: "We are providing as much support as we can to the employees and their families.
"Our thoughts and sympathies are with them."
The leak involved hexamethylenediamine, which is corrosive and can be hazardous in high concentrations.
HSE investigators are looking at the site management systems and interviewing all those involved in the incident.
On Friday a spokesman said: "We have not, as yet, been able to gain access to the plant where the leak occurred as this is still being decontaminated by BASF, but we will be going in there as soon as this work is completed, hopefully later today.
"We cannot say how long our investigations will take but we can say they will be thorough to ensure we get to the bottom of this incident."