Privacy and legal experts across Canada say serious privacy questions need to be asked about an "extraordinary" and "unprecedented" move by Ontario Provincial Police to send text messages to about 7,500 people for information about an unsolved homicide.

Investigators say the mass texting — what they are calling a "digital canvass" — is the high-tech equivalent of knocking on thousands of doors for information.

Privacy experts say Thursday's canvass is different, however, as it relies on information most people might have assumed was private, is potentially far more invasive and raises red flags about what police will do with the data once their investigation is complete.

While the text message push is new here in Canada, police in the United States issued a cellphone alert to millions of phones in the New York and New Jersey areas after the Sept. 17 bombing that injured 29 people.

Police arrested Ahmad Khan Rahami, 28, after they say a resident subsequently alerted police about a man who matched his description.

But the smaller scale and scope of the Ontario investigation has some concerned about when these kinds of searches will be justified.

Texts sent to solve a murder

Police obtained the phone numbers from a cell tower by filing a court order, a step they say they took to solve a murder.

The partially burned body of Frederick John Hatch, a 65-year-old from Toronto, was discovered last December by a motorist on the side of a road near Erin, Ont., a small community about 50 kilometres west of Toronto.

Ontario Provincial Police have released several images showing Frederick John Hatch and what he was wearing before he was killed in December 2015. His body was found near Erin, Ont. (OPP) The body was found just before 6:45 a.m. ET on Dec. 17, 2015 and was located east of 10th Line off Wellington Road 124, just north of Erin.

Hatch had last been seen in Ottawa the day before, at about 1 p.m., inside a Dollar Tree discount store near West Hunt Club and Merivale roads.

Police have posted photos of Hatch. They've offered a reward. They even set up a van showing information about the investigation and have parked it along the route between Erin and Ottawa. But the investigation has stalled.

Det.-Supt. Dave Truax with the OPP criminal investigation unit said police had already done a similar phone tower alert at the town of Erin, Ont., but nowhere near the scale of Thursday's text blast. He said there were so few numbers they either called people directly or knocked on doors.

New leads since texts, says OPP

Truax said two texts — one in English and one in French — will have been sent to all 7,500 phone numbers by Thursday evening.

The numbers were obtained from a cell tower near the Ottawa discount store, in what is known as a "phone tower dump." No names or other contact information was provided, OPP said.

The OPP van highlights a reward and a plea for information into the homicide investigation of Frederick 'John' Hatch. Already, the tactic has helped the investigation move forward, he says.

"We have information today that we didn't have yesterday in relation to John Hatch. We've had calls to our tip line as well," said Truax.

He said the public response has been mixed, with some people calling back to complain, while others saying they thought it was a good idea.

CBC News spoke with two people who received the text, both of whom said they didn't feel it was an invasion of their privacy.

Will police be 'tempted' to do more?

Privacy and technology lawyer David Fraser said the tactic should highlight to the public that "we leave a trail of digital breadcrumbs" every time we use mobile phones.

"As we leave all of these digital breadcrumbs all over the place, I don't think there is any doubt that the police are going to be very tempted to use them much more often," he said.

The concern is that, unlike most police requests to telecommunications companies for phone numbers, this request isn't targetting a group of suspects, but thousands of people who have nothing to do with Hatch's murder, Fraser said.

What happens with the information?

"[We] don't know what are they going to then be doing with this information," Fraser said. "Will it be going into a database? How long are they going to keep it for? Will they make efforts to match those phone numbers against other information? So there's a lot still happening in the shadows [that] I think we need to be concerned about."

We are trying to find a killer. And we think that the public would expect us to take every avenue of investigation available to us in order to do that. - OPP Det.-Supt. Dave Truax

Ann Cavoukian, the province's former information and privacy commissioner, expressed similar concerns, saying it raises some "red flags" that the information obtained in this case could end up being used in another.

"I was hoping that if the individual they texted didn't respond within pre-determined period of time, the metadata would be destroyed, but already that's not the intention. I would want some really strong controls," she said.

Teresa Scassa, a law professor at the University of Ottawa and the Canada Research Chair in Information Law, said while telecommunications companies have pushed back when they felt police were overstepping their bounds in their requests, private citizens shouldn't rely on the telcos to do that every time.

Teresa Scassa, the Canada Research Chair in Information Law, says 'we leave ourselves open to essentially a form of total surveillance' if we don't set limits on what police can access. (CBC ) "If we aren't concerned about what the limits are then we leave ourselves open to essentially a form of total surveillance," she said.

Ottawa criminal defence lawyer Michael Spratt also pointed out police need to be able to prove when justifying the court order that they had a reasonable belief it would turn up evidence.

"Given the broad nature of their request, I think it's more of a fishing expedition and there's less certainty that they will discover evidence," he said.

But lawyers CBC News spoke with say without seeing the court order, it's difficult to know if it oversteps existing privacy laws.

Police aware of privacy concerns

Truax said police are aware of the privacy concerns and had to gauge any information they receive "in relation to advancing the homicide investigation."

"How far do you go? What are the limits? Where does it end? Is it of value? We are trying to find a killer. And we think that the public would expect us to take every avenue of investigation available to us in order to do that," he said.

In this case, Fraser said, because it is a murder investigation, the police method may be a "completely proportional response."

"My big picture concern is... Is this the first of many?" he asked. "Is this only going to be reserved for very serious crimes... or will this become a general tool that could be used in much less serious crimes."