A purposely altered gas line caused the massive explosion in north Edmonton last year that killed three people and damaged 40 homes, said police Tuesday.

Police concluded their investigation into the blast which tore through the Lago Lindo community shortly after 1 p.m. Sunday, June 20, 2010, causing $3.5 million in damage.

“The shock experienced by residents and the magnitude of the damage was comparable to that of a tornado touching down in the neighbourhood,” said acting superintendent Darren Eastcott, who helped co-ordinate the police response to the explosion.

Police said Dwayne Poirier, 46, who died in the explosion, had strangled his common-law wife Jeanne Cathleen [Cathie] Heard, 47, before a natural gas build-up in the home at 18011–91A Street ignited.

Police don't know how or when the line was altered and can't be completely certain that it was Poirer who opened the gas line.

Cathie Heard was killed by her common-law husband before the blast. (Courtesy Lyn Morrison)"What we know is the house was filled with natural gas and in order for that to happen, you have to alter a line," said Eastcott.

Police also don't know if Poirer died by accident or suicide, but Eastcott said it appears he killed Heard and tried to hide it.

According to court documents, the Poirier-Heard relationship was volatile with each alleging frequent physical and verbal abuse.

The blast also killed Craig Huber, 29, and Bradley Winter, 26, whose home next door was also levelled.

The investigation was the most complex he has been involved in, said Eastcott.

"The blast site itself wasn't just within the houses," he said. "The blast went farther north so we had to go into the field. We involved more resources than we would in most investigations to make sure we didn't miss anything."

“We were working with a very unique crime scene, and brought in the necessary technical and forensic expertise to investigate the incident,” said Lorne Pubantz, staff sergeant, EPS homicide section.

“Through the larger team’s efforts, we are able to conclude the homicide investigation, and hopefully, help to bring closure for the victims’ families as well.”

While neighbours were relieved to hear the investigation was concluded, some remain frustrated at the pace and extent of the cleanup.

"If you go down there and take a picture, it's terrible," Pat Kavanaugh said. "The garbage ... it's from the explosion still. They have not cleaned it up properly."

The cleanup was delayed for months because it wasn't clear who would pay for it since both homeowners were dead.

A deal was reached in November, which will see the city charge the cost to the land's property tax bill, meaning the cost will be paid by whoever buys the land.

Ryan Pleckaitis, Edmonton's director of complaints and investigations, said while the city has cleaned up the property to the best of its ability, an officer will pay another visit to see what additional work may be required.