As the FDNY sifts through the remains of two Grand Street buildings where a seven-alarm fire broke out on Sunday night, killing one person and injuring dozens, they apparently do not think it was arson. A source tells the Daily News the fire's cause may have been electrical, "We believe it was smoldering for a long time. Once it reached the building's inner shafts, it exploded." The building "contained several dumbwaiter shafts that helped the flames spread quickly." The buildings, 283 and 285-287 Grand Street, contain both market-rate and rent-stabilized apartments and had been on the market. Residents have complained about lack of hot water and heat and the buildings had many violations.