Police confirmed Rick F. Nelson died just hours after being taken to Harborview Medical Center with a self-inflicted gunshot wound on Thursday afternoon.
Nelson, 50, of Issaquah was arrested early Thursday morning, booked into jail, then released on his own cognizance.
Approximately six hours after he was released, Nelson was found shot on the John Wayne Trail near Rattlesnake Lake. He was taken to Harborview Medical Center where he was later pronounced dead.
Seattle Police Chief John Diaz said the department launched a criminal investigation last summer after fellow officers raised concerns about Nelson showing an "inordinate interest" in drug evidence that were brought in.
"It was a hunch," said Diaz, adding such investigations, which are referred to as "integrity tests" within the department, are routine when concerns are raised.
As a part of the investigation, officials carried out an undercover sting on Wednesday night when an officer from an outside agency handed Nelson a bag filled with cocaine. Department officials were surveilling Nelson to see whether he would follow proper protocol for handling drug evidence.
"Unfortunately, he did not do that," said Assistant Police Chief Nick Metz. When officers arrested Nelson several hours later, they found some of the drugs on his person.
Metz, calling the event "a tremendous tragedy," said Nelson had been a "friend to many" in the department.
"It's been a heartbreaking 24 hours, to say the least," he said.
An officer who previously worked as Nelson's patrol partner said the news came as a surprise.
"(For) those of us who knew him and even those of who didn't, it's just a shock, a tragic ending that nobody would have expected," said officer Brian Guenther. "Just a very somber and sober day."
Guenther, who said he'd known Nelson for 20 years, said he did not know whether Nelson struggled with a drug problem.
"All I know is he was well-liked by everybody at the precinct, and just a good-hearted person," he said.
Nelson, a 21-year veteran of the force, worked out of the South Precinct for his entire career. He is survived by his wife and two teenage children.