London police are treating an explosion at an east-end house that seriously injured one man as suspicious.

Concerns about structural damage to the single-storey home at 1335 Hamilton Rd. kept investigators out for two days following the Monday afternoon explosion, which also killed a dog.

An inspector from the Office of the Ontario Fire Marshal was able to go inside the house Wednesday, deputy fire chief Jack Burt said Thursday.

“We intend to wrap that investigation up later today,” Burt said, as fire officials remained on the scene for a fourth day.

“At this point we’re still trying to determine the origin, cause and circumstances of the fire.”

Police are leading a joint investigation with the fire marshal and the London fire department. The probe has been assigned to the street crimes unit, police said Thursday.

The explosion left the front west side of the home badly damaged. One man was taken to hospital with burns, while two other occupants escaped unharmed.

The injured man was listed in serious condition at the hospital Thursday, police said.

The owner of the home, who said he’s been in Mexico for the past week, told The Free Press that his tenant had been allowing a couple to stay at the unit without permission.

dcarruthers@postmedia.com