A missing 5-year-old boy with autism has been found dead in a river in Washington state after he vanished in September, authorities said.

Lucian Munguia’s remains were discovered Thursday by a resident who was walking their dog near the Yakima River, the Yakima Police Department said.

An autopsy conducted Friday determined the remains belonged to the non-verbal boy, who was last seen at Sarg Hubbard Park in Yakima on Sept. 10.

Following a comparison of dental records, police investigators and the county coroner’s office confirmed the remains are those of the 5-year-old.

Yakima County Coroner Jim Curtice said Friday that the boy had been the victim of an accidental drowning in the river, and there was no evidence of foul play.

Police have also said they do not believe any criminal acts led to the boy’s death. They will provide additional details once DNA testing is completed.

Following Lucian’s disappearance on Sept. 10, the community had rallied together to try to find the boy, whose family reported him missing to authorities 15 minutes after they first began looking for him, spurring a massive search effort, the Yakima Herald-Republic reported.

Yakima Deputy Fire Chief D.J. Goldsmith, who participated in the search efforts, said the nearly four-month-long search for Lucian was not abnormal given the nature of the river.

“Even though we give 100% effort to locate things, including using infrared drones, there are so many tributaries and multiple places where things can get hung up,” Goldsmith said.

Authorities have not shared how his family lost track of him. Search and rescue crews from multiple nearby counties scoured the park, river and surrounding area for the boy.

An FBI dive team did a thorough search of the park’s pond and other nearby bodies of water. Even after authorities scaled back their search efforts, volunteers and Lucian’s family continued to search for the boy.

Throughout the search, a $10,000 reward was offered for information to find Lucian.

Police offered their condolences to the boy’s loved ones in the wake of the tragic discovery.

“Our thoughts and prayers are with the family and friends of Lucian,” police said.