A handful of indigenous activists and researchers were celebrating a birthday huddled around a small pit fire when they noticed the flames devouring a building a few dozen yards away.

“It’s the museum that’s on fire!” said José Urutau Guajajara, a member of the Tenetehára-Guajajara tribe who had been researching his people’s heritage in the archives of Brazil’s National Museum for more than a decade. “We can still manage to put it out with buckets.”

By the time they reached the centuries-old palace, home to the world’s largest archive of indigenous Brazilian culture and history, flames had gutted the building’s core and a dense column of smoke towered above it.

Twice, Mr. Guajajara tried to run into the building and was held back by guards. After that, his friends restrained him. Together they watched as hundreds of thousands of documents, artifacts and artworks were reduced to ashes on the night of Sept. 2.