Police in Washington state have arrested two suspects in the murder of a father of two found dumped in a shallow grave near the Nisqually River. 

The Thurston County Sheriff's Office has been investigating the homicide of Robert Erwin, a transient in his early 40s, whose body was found buried in a shallow grave near the Nisqually River June 4. 

"Today, with the assistance of Yelm Police, Tumwater Police, and the Thurston County Narcotics Task Force, TCSO detectives have arrested and booked a man and woman in their mid-30's from the Yelm area for Rendering Criminal Assistance, Tampering with Physical Evidence and Unlawful Disposal of Human Remains," Lt. Mike Brooks said in a press release.

Erwin reportedly struggled with mental health issues and addiction, Amanda Douglass, Erwin's sister, told FOX 13 Seattle. Despite his struggles, Douglass said he was loved by his family, including his two sons. 

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Robert Irwin in a brown jacket

The investigation into the murder of Robert Erwin continues in Washington state after officials arrest two potential suspects.  (FOX 13 Seattle/Family handout)

Douglass told FOX 13 her brother's case was initially treated as a missing person case and that the sheriff's office did not have reason to believe it was a homicide case. 

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Robert Irwin playing guitar

Authorities have arrested two suspects in the murder of Robert Erwin, who was found buried in a shallow grave near the Nisqually River.  (FOX 13 Seattle/Family handout)

"It’s a tremendous thing to navigate. To be told that your loved one was brutally murdered and just dumped," Douglass told FOX 13 after news of her brother's murder broke. "My brother wasn’t just a homeless scumbag living on the streets." 

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Photo of Robert Irwin holding his two sons

A family is searching for answers after Robert Erwin was found murdered and buried in a shallow grave in Yelm, Wash. (FOX 13 Seattle/Family handout)

The sheriff's office has not released further details about the suspects or a motive for the murder.  

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The investigation remains ongoing, and officials urge anyone with information about the case to call 360-704-2740.  

Fox News Digital reached out to the Thurston County Sheriff's Office for comment.