Fire investigators concluded an electrical cord running under a fold-out couch caused the fire two weeks ago that killed two people in Chelmsford, the state fire marshal’s office said Thursday. Susan Astle, 67, and Paul Deslauriers, 77, died in the Jan. 5 Woodcrest Condominiums fire, and a 7-year-old boy is still hospitalized. Twenty-one apartments were destroyed by the fire and 31 people were left homeless. The building did not have a sprinkler system, which might have mitigated some of the damage and loss of life, because it was constructed before sprinklers were required in buildings of that type, a Department of Fire Services statement said.Todd Feathers can be reached at firstname.lastname@example.org. Follow him on Twitter at @ToddFeathers.