FBI agents and state and local police spent the day yesterday digging up land around a Peabody house of horrors where last February a couple were murdered and partially buried in a cellar under garbage and debris.“During the investigation into the double homicide that took place at this location last February, information was obtained that necessitates a further search of the property,” said Carrie Kimball-Monahan, spokeswoman for Essex District Attorney Jonathan Blodgett, in a statement.