Investigators probing the horrific Hopkinton house explosion that killed two young sisters in July have identified a faulty mechanical part in the dwelling's natural gas delivery system as the likely cause of the deadly blast, the Herald has learned.

According to sources close to the investigation, a "sleeve-like" device that brought gas through the three-story dwelling's cement foundation from an outside gas line is believed to have failed, sparking the massive July 24 explosion that collapsed the building and killed 5-year-old Violet and 4-year-old Iris Carey.

"The part failed," said one source. "There are still more tests to be done, but that's going to end up being the cause."

State Fire Marshal Stephen Coan would not confirm the part is the cause of the blast, but said the investigation has reached the point "that information can now be shared with the (federal) Department of Telecommunication and Energy, who have regulatory authority" over the investigation.

"The cause is very much still under investigation and no final determination has been made," Coan added.

The girls' parents, Heath and Tara Carey, who survived the explosion along with eight other building residents, declined comment last night.

Their attorney, John Wozniak, lost an August Worcester Superior Court bid to have experts hired by the Careys included in the state's investigation.