Mexico City, Mexico (CNN) -- A missing 4-year-old Mexican girl, whose body was found under a mattress in her bedroom, died of asphyxiation, officials said.
The March 22 disappearance of Paulette Gebara Farah drew great attention throughout Mexico, and officials said they assigned 30 investigators to the case. Web pages from friends of the family and others were posted on the Internet, asking for help finding her.
The girl's mother, Lisette Farah, is the prime suspect, said Alberto Bazbaz, attorney general for the state of Mexico. The mother has not been charged.
"She is the only suspect," Bazbaz said at a news conference Wednesday. "I have no doubt that this is a homicide investigation. At this moment, we can say that Lisette is a suspect. In addition to her, we are investigating the level of knowledge of others involved."
Farah and her husband, Mauricio Gebara, and two nannies were detained Monday after authorities said they found discrepancies in their stories. No one has been charged, Bazbaz said.
Bazbaz said undercover recordings of conversations between the mother and her husband and older daughter also led investigators to suspect her.
Investigators found Paulette's body in her bedroom early Wednesday in a space between the bed frame and the mattress, the attorney general's office said. Her body was covered by sheets and a blanket from her bed, Bazbaz said.
Photographs at Wednesday's news conference showed the little girl lying on her side, with red pajama bottoms and a blue sweater, the state-run Notimex news agency reported.
Authorities did not notice anything amiss in the room until Tuesday when they detected a faint odor of a decomposing body, Bazbaz said.
"Her death was ultimately caused by ... asphyxiation that obstructed her respiratory airways and compressed her abdominal thorax," Bazbaz said.
Her parents said they put Paulette to bed Sunday night and she was missing Monday morning.
Paulette could not talk and needed help to walk.
Accompanied by Sandra Yadeum, a psychiatrist for the attorney general's office, Bazbaz said the mother apparently suffers from some type of personality disorder.
Yadeum described Farah as a lawyer who is "very intelligent, very capable, very astute" but who is cold and without emotional attachment.
But Yadeum said that "does not automatically convert her into a delinquent."
Bazbaz said the case is particular and complex and that the personalities of those involved "also are atypical."
Salvador Cuevas, an attorney for nannies Erika and Martha Casimiro, said the two sisters have worked for the family for seven years.
Their father, Jesus Casimiro, told TV Azteca his daughters have not done anything wrong.
The family lives in Huixquilucan, a town in the Mexico City metro area.
The family had returned from a three-day vacation on March 21, the day before Paulette's disappearance was reported.
Bazbaz said Wednesday officials have not determined when or where Paulette died.