Fire most likely started by short-circuit
Health Minister Nelu Tataru told Romanian media the fire at Piatra Neamt Regional Emergency Hospital was "most likely triggered by a short-circuit".
Romania's national prosecutor's office said it would investigate the blaze as a possible case of criminal manslaughter.
The prosecutor who led the probe into a 2015 blaze that killed 64 people at an underground nightclub in Romania's capital, Bucharest, was assigned to lead the new investigation, the office said.
The fire at the Colective nightclub sparked massive protests that forced the Romanian government to relinquish office.
The government was led at the time by the left-leaning Social Democrat Party, known as PSD.
The PSD hopes to reclaim power in the next general election, set for December 6.
It is almost certain to use the Saturday fire in Piatra Neamt in its attacks against the centre-right National Liberal party, or NLP, which has controlled Romania's minority government since last October.
The minority government has been struggling to contain the economic fallout from the coronavirus pandemic by curbing social spending, and the tragic death of patients in a public Covid-19 hospital is likely to add to the ruling party's woes ahead of the election.
News outlets reported hospital has long been poorly managed
Eight government-appointed managers oversaw the facility in the last year.
The hospital's current manager, Lucian Micu, was appointed just three weeks ago after his predecessor resigned over the poor treatment of patients.
The resignation followed media reports of patients, including many suspected to have Covid-19, being forced to wait outside in the cold to see a doctor.
Micu said on Saturday that a doctor on duty tried to save the patients from the flames and was in critical condition with first- and second-degree burns covering 80 per cent of his body.
Another doctor and two nurses also were being treated for burns.
Piatra Neamt is about 353km north of Romania's capital, Bucharest.
