TORONTO -- The task of searching for human remains in the case of an alleged serial killer is so daunting that Toronto police say cadaver dogs from neighbouring forces are now assisting with the massive and expanding investigation.

Hundreds of police officers have been going through at least 30 properties connected to Bruce McArthur, a 66-year-old landscaper accused of killing five men and hiding skeletal remains in planter boxes.

So far police have uncovered the remains of six people, and have identified one of them -- 49-year-old Andrew Kinsman, who disappeared from the city's gay village in June 2017.

Toronto police said they were working with the RCMP to sift through the National Centre for Missing Persons and Unidentified Remains database in an effort to identify the other remains.

The force expects to expand its search geographically outside the Toronto area eventually, but for now they said the focus remains in the city.

"It's a daunting task, but we are going to do it," said Toronto police spokeswoman Meaghan Gray.

Even the dogs are exhausted, she said, so cadaver dogs from the Ontario Provincial Police, York and Peel forces have been helping out.

"Our dogs are getting tired and they needed a rest and we had to keep the investigation going so we asked for help," Gray said. "The dogs really do need to rest their noses. They get less effective the more they work."

McArthur has been charged with five counts of first-degree murder for the death of Kinsman and the presumed deaths of Selim Esen, 44, Majeed Kayhan, 58, and Soroush Mahmudi, 50, as well as the death of Dean Lisowick, who was either 43 or 44. All five men are believed to have had ties to the LGBT community.

Police have said they expect to lay more charges against McArthur.

"Right now we're focused on finding remains and identifying those remains," said Gray. "Most of our partnerships are with units that can help us do that."

On Friday, Dr. Kathy Gruspier with the Ontario Forensic Pathology Service and her team began digging into the backyard of the home in midtown Toronto where all the human remains have been found, Gray said.

The lead detective on the case, Det. Sgt. Hank Idsinga, spoke Thursday about the scope of the investigation.

"It is getting bigger and we are getting more resources the further we go along," he said. "It's going to be a very, very extensive investigation as we go."

Ontario Provincial Police have helped out with their ground penetrating radar device in the backyard. Several spots of interest have been identified, Idsinga said.

Renee Willmon, a forensic anthropologist who has worked on other cases with both Toronto and provincial police, said the device allows police to determine disturbances in the soil or if something is buried.

"These are the worst conditions to be excavating in given how cold it is," Willmon said. "You don't want to be pulling up chunks of soil. It needs to be soft enough work with hand tools like a trowel and work is only done centimetres at a time. This could take quite a while."

In addition to the scores of tips that are coming in from the public, numerous police forces have reached out to Toronto inquiring about other missing persons cases, Gray said.

"We will be working with those jurisdictions to review those cases, but we're focusing on Toronto first," she said.