Bosma's remains were later found burned beyond recognition at a farm belonging to one of the suspects. But Hamilton police said they still don't know why he was killed.
"I don't have an answer to that question today," Supt. Dan Kinsella said in announcing the arrest of a second suspect in the case.
Police say they now have in custody the two men they allege Bosma took for a test drive in the truck he had posted for sale online.
Mark Smich, 25, was arrested on foot in his hometown of Oakville, Ont., Wednesday morning, about an hour before a memorial service for Bosma began in Hamilton.
A Toronto man, 27-year-old Dellen Millard, was already facing charges of first-degree murder, forcible confinement and theft of a vehicle. His lawyer has said he will plead not guilty.
"We believe the danger has subsided as it relates to this case," Kinsella said.
Police are still looking for at least one other person, as a dark blue GMC Yukon belonging to Millard was seen on video following Bosma's truck as he left his home on the test drive. Kinsella urged that person to turn themselves in.
Millard and Smich know each other, Kinsella said, though he wouldn't say how.
Smich is known to police, Kinsella said, but wouldn't elaborate. He is to appear in court Thursday in Hamilton, where he'll face a charge of first-degree murder.
As many as 120 investigators have been working on the case, executing more than 15 search warrants and production orders and following up on more than 700 tips, Kinsella said.
"This is a very time-consuming process," he said, adding the ongoing investigation would force him to remain tight-lipped on many aspects of the case.
Journalists from more than half a dozen outlets peppered him with questions, but Kinsella cut the news conference short by walking out of the room after answering only a handful.
Officers scoured a wooded area behind an Oakville home listed under the name A. Smich late Wednesday afternoon while neighbours gaped from the sidewalk. A police cruiser was parked in front of the brick home.
Bosma's wife Sharlene last saw him the night of May 6 when he went for a test drive in the 2007 Dodge Ram pickup truck that he posted for sale online. A man called Bosma and arranged to go on a test drive that night, police said. Bosma never came home.
Bosma's cellphone was found three days later in an industrial area of the neighbouring city of Brantford.
Police believe he was killed the same night he went missing, though they haven't yet said if he was killed in the truck, on the farm where his remains were found, or elsewhere.
Bosma's remains were found about a week later on property owned by Millard in Waterloo Region, about 50 kilometres from Bosma's home. Investigators seized an incinerator found on the property, but they won't say if it had been used or not.
Police also searched a Millard Air hangar at the Waterloo regional airport.
Millard's family is well-known in the area and has a storied past in aviation. When Millard was 14 he became the youngest Canadian to fly solo in both an airplane and a helicopter. His grandfather and father were also pilots.
In light of Millard's arrest, Toronto police have reopened investigations into his father's death, which had been considered a suicide, and the disappearance of Laura Babcock, a Toronto woman with whom he was reportedly linked.
Millard's lawyer, Deepak Paradkar, said his client hasn't been charged in either of the Toronto investigations and isn't required to address accusations "for which we have nothing but conjecture."
Millard is set to appear next in court by video remand on June 13.