Lehi police were led to the suspect by "information and evidence" Swenson could not further detail, pending filing of charges.
Swenson said charges, including animal cruelty, were being screened with the Lehi city attorney's office.
The suspect, while admitting to the fatal poisoning of a 2-year-old dog, maintains he does not know how the second, 8-9-year-old dog became ill. Both animals reportedly suffered seizures consistent with strychnine poisoning, though final results of laboratory tests were still pending Tuesday.
Swenson said charges would "definitely" be sought in connection with the first, fatal poisoning, and police have submitted stool samples from the second, still-living dog for testing to determine if additional charges are warranted.
"It will take a week or two for us to get results back on those tests," Swenson said.
Both dogs were in fenced yards at residences within the same cul-de-sac in northwest Lehi. The first dog suffered a series of ultimately fatal seizures beginning about 6 a.m. on July 1; the second suffered a seizure four hours later, but survived.
The Humane Society of Utah had offered a $3,000 reward for information leading to an arrest in the case, but spokesman Carl Arky said that as of Tuesday, no one had claimed it.