After nearly two months of digging through a Victorville landfill, authorities said Tuesday they have called off the search for remains of a 6-year-old boy believed to have been killed by his mother and aunt.

Investigators first discovered Duke Flores of Apple Valley was missing after his grandmother requested a welfare check on April 25, and his mom, 30-year-old Jackee Raquel Contreras, told them she hadn’t seen him for about two weeks, the San Bernardino County Sheriff’s Department said in a news release.

Contreras was arrested that day on suspicion of child neglect for not reporting the boy was missing, but two days later she was booked on suspicion of murder after being interviewed by detectives, officials said.

Her twin sister, Jennifer Rachel Contreras, was also arrested on suspicion of murder following interviews with family members, deputies said.

Other relatives have said the boy had autism and wandered away in the past.

The sisters pleaded not guilty to murder charges, but investigators believe they discarded the 6-year-old’s body in a dumpster near the family home on Cherokee Avenue.

Crews began searching a landfill in Victorville on April 29, and in the ensuing weeks they sifted through roughly 13 million to 14 million pounds of refuse.

The operation concluded last Friday, June 21, with no sign of Flores’ remains.

“This is not the outcome we had hoped for, but we knew going into this search that there was a chance we would not locate Duke,” Sheriff John McMahon said in a statement. “I am extremely proud of the dedication shown by every person who continued to show up day after day, desperately hoping to locate the young boy.”

Coroner’s and district attorney’s investigators as well as landfill personnel assisted homicide detectives, K-9s, deputies and volunteers from the Sheriff’s Department in the recovery effort.

The Contreras twins were both being held on $1 million bail and due to return to court Wednesday.