LAS VEGAS — Police found the body of a 3-year-old boy Monday at a remote area near Lake Mead a day after distributing a photo showing him with a wide smile in a pirate costume.

His 20-year-old mother Cassie Smith and her boyfriend, 40-year-old Joshua Oxford, have been arrested on suspicion of child abuse in connection with the treatment of the 3-year-old's younger brother and will likely also be charged with murder in the death of the older boy, police said.

Officers became suspicious Sunday when Smith reported her toddler son Daniel Theriot missing at a Las Vegas park. Police distributed the photo of the blonde boy as part of their search.

"I will tell you it is absolutely disturbing on how the child, we believe, was killed," police Lt. Ray Spencer said during a news briefing.

Spencer ruled out the possibility that the boy's death could have been an accident and said authorities believe he was killed at least 12 hours before his mother reported him missing.

Police say the boy's younger brother, a 2-year-old, was taken from the family home and showed signs of physical abuse. He's in the custody of the child services department.

Smith's story unraveled not long after police began an exhaustive search of the park where she reported him missing, Spencer said.

He said no witnesses had seen them at the park, and it didn't make sense that they were there in the first place, since there is another park within walking distance of the family's home.

The killing came less than two weeks after Las Vegas police found a 3-year-old girl dead in a duffel bag inside her family's apartment.

Police say 29-year-old Aisha Yvonne Thomas, the girl's mother, lied about her daughter going missing. An autopsy found she died of blunt force injuries before her body was found Aug. 23.

Thomas is being held without bond and will plead not guilty, according to her attorney.

Smith and Oxford were arrested Monday and no attorney is listed for them on booking records.

Spencer said both cases were deeply disturbing.

"If you find yourself in a situation where you cannot care for your child, you can take your child to a fire station, you can take your child to a hospital, but this is absolutely, I mean, it's hard to deal with," Spencer said.