RIO DE JANEIRO - Forensic investigators and researchers awaited access on Tuesday to the National Museum in Rio de Janeiro, gutted in a fire, to find out how the blaze began and what remains of the 20 million artifacts that made the museum one of the most important in Latin America.

After the fire tore through the museum on Sunday, engineers were doing tests on the structure to make sure it wouldn't collapse. Authorities had expressed concern on Monday that internal walls and parts of the roof were weak.

The museum held Latin America's largest collection of historical and scientific artifacts, and officials suggested that the damage could be catastrophic, with one official telling a Brazilian news outlet that as much as 90 percent may have been destroyed.

Fire had spread quickly through the wood interiors of the three-story building, fueled by a vast collection of documents.

The cause of the fire was not known. Federal police will investigate since the museum was part of the Federal University of Rio de Janeiro. But protesters, commentators and museum directors said years of government neglect had left the museum so underfunded that its staff had turn to crowdfunding sites to open exhibitions.

In another example of struggling public services, firefighters initially struggled to contain it because the hydrants closest to the museum did not work. Instead, trucks had to gather water from a nearby lake.