BOULDER, Colo. -- A 6-month-old Superior baby died of suffocation, and his death has been ruled a homicide, the Boulder County Coroner's Office said Wednesday.

Rylan Rochester was pronounced dead on June 1 at Avista Adventist Hospital. The coroner's office had been waiting on the results of lab tests before issuing a final ruling on the infant's cause of death.

The boy's mother, Stephanie Rochester, faces charges of first-degree murder and child abuse in the case.

According to her arrest affidavit, Stephanie told a police detective that she thought her son "was showing signs of autism."

"She wanted to kill herself, but did not want to burden her husband with the baby inflicted with autism," the affidavit stated. Her husband also told police that she had been depressed for some time and was taking Zoloft.

Stephanie worked for two years as a counselor at The Children's Hospital and worked with children with autism, according to the affidavit.

Stephanie told police that when they got home the evening of June 1, she put a plastic bag over the infant's head and then put a blanket on his face as he lay in his crib.

"She said that she had conducted research on the computer and read that carbon monoxide poisoning deaths don’t hurt and you just go to sleep. She did not want Rylan to hurt," the affidavit said.

According to the affidavit, Stephanie said she removed the items a minute later and went downstairs to eat dinner with her husband.

During dinner, the two talked about selling their house and about how "they wanted to have fun in life. Stephanie said that she knew they would not have fun while they were caring for a severely autistic child," the affidavit said.

After dinner, Stephanie went back upstairs and put three baby blankets on top of her son's face, the arrest affidavit stated. She checked on him an hour later, took the blankets off his head and he was still breathing, but his eyes were open and he was unresponsive.

After the couple went to bed, Stephanie told detectives she could hear her son whimpering in his crib at about 2:30 a.m. so she got up and put two folded adult blankets on the baby's head, "so that he would die," the affidavit quoted her as saying.

The next morning, Stephanie got up at 6 a.m., and thought "about going into the garage and turning on the car to kill herself, but instead went into the baby’s room. She took the blankets off of Rylan’s head and "just lost it" when she realized what she had done, the affidavit said.

The couple rushed the boy to the hospital where he was pronounced dead.

The husband, who was not charged in the case, has since filed for a divorce from his wife.

Stephanie is being held without bond and faces a Sept. 8 preliminary hearing.