A Tucson couple was arrested Wednesday after human remains believed to be those of their 3-year-old son were found in a plastic toy chest in the backyard of a home where the family had been living.

The remains are believed to be those of Roman Barreras and were discovered Tuesday by a landlord who was cleaning the home as the family was in the process of moving out, Tucson police said.

The boy's 39-year-old mother, Raquel Barreras, was arrested on suspicion of first-degree murder and child abuse, according to police. They said the boy's father, 45-year-old Martin Barreras, also was booked on suspicion of child abuse.

Tucson police spokesman Sgt. Pete Dugan said authorities were trying to determine how and when the child died. The cause of death will be determined by medical examiners.

Police were unable to tell whether there were signs of abuse on the decomposing body.

Dugan declined to say whether family members knew of abuse of the child, but said relatives had raised questions about where the boy was and were given various answers.

The couple has four other children ranging in age from 4 to 19.

The three youngest children who are minors have been turned over Child Protective Services, authorities said.

Dugan said there didn't appear to be any signs of abuse on the other children, but the children suffered from neglect, such as not attending school regularly.

Tucson Police Chief Roberto Villasenor said the family had previous contact with CPS and the agency had taken the children away on previous occasions but returned them to the parents.

CPS officials confirmed prior agency involvement with the family — the most recent being in August 2012 — but couldn't immediately provide any details.

It's was unclear whether Raquel and Martin Barreras had a lawyer by Wednesday night. The couple doesn't have a listed phone number.

Dugan said the remains weren't those of 6-year-old Isabel Mercedes Celis of Tucson. Authorities have been searching for the girl since her father reported her missing in April 2012.