The three-alarm blaze that ravaged the Georgetown Neighborhood Library on Monday was accidentally caused, likely by “construction workers using a mechanical heat device,” D.C. fire department spokesman Alan Etter said Thursday. Repairs to the 72-year-old branch are expected to approach $20 million, officials announced this week. The children’s room was a total loss, but the Peabody Room, where the library’s rare collectibles were stored, remained somewhat intact. Roughly 80 percent of the room’s documents, artwork, maps and ceramics were believed to be saved.