A nursing assistant looking after coronavirus patients has died.
John Alagos, 24, became ill and died on Friday after working at Watford General Hospital in Hertfordshire.
His mother Gina Gustilo told the BBC the family were waiting to hear whether he had tested positive for Covid-19.
The hospital closed to new patients on Saturday due to problems with its oxygen equipment, but has now reopened. It said it ensured staff had the right level of protection when working.
Tracey Carter, chief nurse at the hospital, said: "John was very popular and will be missed greatly by his colleagues."
Mrs Gustilo, who is an NHS mental health nurse and lived with her son, said she had shown no symptoms of the virus but was self-isolating at home.
In a statement issued after Mr Alagos' death, Ms Carter added: "Our staff are fully briefed on the symptoms of Covid-19 and we would never expect anyone to remain at work if they were showing these symptoms or indeed were unwell in any way.
"We have always kept our staff updated on the latest PPE guidance to make sure they have the right level of protection for where they are working."
The hospital said the decision on Saturday to declare a critical incident was taken as a "result of a technical issue with our hospital's oxygen equipment".
'Safe oxygen levels'
Later that evening the hospital said the problems had been resolved and it lifted the critical incident.
A safe level of oxygen was maintained throughout the duration of the incident, said the West Hertfordshire Hospitals NHS Trust, which runs the hospital.
As of 17:00 BST on Thursday, 29 people had died at the NHS trust's hospitals after being diagnosed with Covid-19.
The trust is responsible for Watford General as well as Hemel Hempstead and St Albans City hospitals.
Current NHS advice tells people with coronavirus or suspected symptoms to avoid hospitals and other medical settings like pharmacies.