Federal police say an air conditioning unit is the "primary cause" of the fire that destroyed Brazil's National Museum in Rio de Janeiro.

Fire experts on Thursday presented the conclusions of a seven-month investigation into the Sept. 2 fire, which began in the museum's auditorium and quickly spread to the rest of the building, destroying most of its 20 million artifacts.

Investigators say they don't know what ignited the fire inside the unit, but stressed that aside from fire extinguishers, the museum lacked most recommended fire protection devices, such as hoses, sufficient water sprinklers and fire doors.

According to the Open Accounts nonprofit that tracks spending, the museum had spent only $4,000 on safety equipment from 2015 to 2017.