Ontario police are broadcasting thousands of text messages to phones used close to the site of a murder.
Police hope the messages will bring forward new evidence and eyewitnesses to the murder of John Hatch last year.
The phones have been identified as being in use on 16 December close to the route Mr Hatch travelled on the night he was killed.
About 7,500 people are expected to receive the messages asking them to contact police.
Dangerous offender
The messages will also direct people to a website to answer a few questions that the Ontario Provincial Police (OPP) said would help its officers.
"We believe this new investigative technique will reach witnesses," said OPP detective superintendent Dave Traux in a statement.
"They may help us solve it by volunteering information and potentially remove a dangerous offender from society before they harm someone else," he said.
The OPP said it used a court order to discover the numbers of all the active phones known to have been used last year in the vicinity of the murder location in Nepean, Ontario.
Ontario police have used the mass-messaging technique, known as a tower dump, before now, but its use was challenged in Canadian courts after one local force applied to use it to contact more than 100,000 people.
After that, the courts ruled that any requests to use tower dumps had to minimise any potential invasion of privacy.
OPP said its court order only sought phone numbers rather than names or other personal information about the owners of the handsets.
A C$50,000 (£30,000) reward for information leading to the arrest of Mr Hatch's killer was put up in June by Ontario's government. In August, police used a van bearing Mr Hatch's image to canvas the route he is believed to have travelled.