"Some physical materials were in the scanning center because they were being digitized, but most were in a separate locked room or in our physical archive and were not lost," the San Francisco-based archive said in a statement.
"Of those materials we did unfortunately lose, about half had already been digitized. We are working with our library partners now to assess."
The nonprofit archive was founded in 1996 and helps researchers, historians, scholars and others find historical collections that exist in digital format.
"This episode has reminded us that digitizing and making copies are good strategies for both access and preservation," the statement said, adding that the damage to equipment from Tuesday's fire was estimated at $600,000, not including building repairs.
"We have copies of the data in the Internet Archive in multiple locations, so even if our main building had been involved in the fire we still would not have lost the amazing content we have all worked so hard to collect."
The archive has some 364 billion archived Web pages along with texts, audio, moving images and software.
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