Guelph police are investigating the latest in a string of letter bombs sent to homes in southern Ontario, a device described by the person who found it as large enough to do serious harm.

Officials confirmed Thursday that a letter sent to John Becker's home in the city contained a "homemade explosive device."

Becker, a self-employed home renovator, found the Canada Post Xpresspost envelope around 3 p.m. Wednesday at the rear of his home but didn't open it.

He instead called police, who brought in a bomb disposal unit and ordered the evacuation of several homes near the intersection of Paisley and Edinburgh roads.

Media reports say the letter bomb was detonated at the scene, sending projectiles flying and blowing a hole about the size of a large book in the deck.

Becker told the Guelph Mercury newspaper the device contained explosive material that would have taken his head off, if it had exploded.

"The disposal unit, based on their initial look at the material after it was detonated, had the opinion that it certainly had a charge," said Sgt. Cate Welsh of Guelph police.

"It has not been determined whether it was delivered to that address deliberately or randomly."

Welsh said Guelph police are in contact with Toronto police about the incident. Two letter bombs have recently been sent to Toronto residents, one causing a minor injury. Both were inside a bubble-wrapped envelope.

One bomb, found in a mailbox in the area of Victoria Park and Lawrence avenues on Aug. 11, exploded as it was opened, injuring a man's hands.

A man who found a similar package in his mailbox in the Sheppard Avenue-Yonge Street area on Aug. 19 did not open it after noticing a petroleum-type odour coming from the letter.

With files from the Canadian Press