It started with a debris-clogged paper mask that fell onto the desk of Jan Ramirez on the afternoon of September 11, 2001. A friend had used it to help him breathe while fleeing downtown Manhattan.

“That dust mask is going to be an important artifact some day,” he told Ms. Ramirez.

More than five years later, the mask has become a museum piece and one small part of the largest records trove ever assembled to document an event.

Millions of pieces of paper documenting government investigations, BlackBerry messages written by survivors escaping the twin towers, children’s finger-paintings, and family photographs are also part of the archive, stored in many different places including state offices, museums, and on the Internet.

Saving all things related to the attacks of September 11, 2001 was a mission embraced from the time of the attacks by professional archivists and grassroots collectors, helped by live televised images of the event and thousands of Internet communities that followed the aftermath.

Archivists, who immediately set out to compile the most complete picture ever of one historic event, are already planning for decades ahead, trying to figure out how to preserve everything.

“There’s a preciousness that comes attached to anything left concrete from this event,” she said. “I think people seem to feel that it was sort of almost this sacred stewardship they have taken on in holding this material.”