Brewer said Robertson's known associates aren't known for being openly cooperative with police and "they have been keeping their mouths shut pretty good."
Brewer said investigators, who had initially looked at the death as a possible homicide, now were calling it "a suspicious, unattended death."
"The Medical Examiner's autopsy called the cause 'undetermined.' There was no physical damage like broken bones or a fractured skull, gunshot or knife wounds," Brewer added.
He confirmed that among several theories about how Robertson met his end was that he may have died in an acquaintance's home and was then stuffed into the sewer to avoid contact with police.
"He had cancer and people tell us he had given up, wasn't taking his meds or chemo. He was giving away all his stuff, he was ready to go from what we we've been told," Brewer said.
"He could have died from natural causes from the cancer," he added. "But putting himself down there? I will never believe that."
Robertson, 38, of Price, was reported missing on Nov. 29, 2011, by his mother, Emma Robertson. His remains were found in mid-December by a public works crew doing routine maintenance inspections. When workers lifted a manhole cover, they saw the body below and called 911.
Initial identification was stymied by the condition of the body, which had been subjected to the effects of winter weather and the flow of water in the sewer system, as well as the lack of any identifying documents on the body. Based on records of his past medical and surgical history, an autopsy identified Robertson.
The case continues to be investigated as a homicide, but Brewer acknowledged the probe had hit a plateau. Investigators need help from potential witnesses, especially those who may have seen the victim, or who could say whom he was with in late November 2011.
Brewer said tips can be kept anonymous if witnesses fear possible retaliation for coming forward.
Anyone with information is urged to call the sheriff's office at 435-636-3281.