AZTEC — A man accused of molesting a 4-year-old girl while the girl's mother smoked methamphetamine in another room went on trial Monday.

Prosecutors say defendant Ramon Martinez, 54, preyed on the girl in his own home in September 2011. While the mother was using meth and the girl was sleeping on a couch, Martinez sexually assaulted the child, Chief Deputy District Attorney Dustin O'Brien said in his opening statement.

"How a girl could get in that situation ... is not a very good story, and it starts with a mom who was not being a very good mom," O'Brien said.

But O'Brien told the jury that Martinez, not the mother, should be held accountable for the assault.

"This case is not about a bad mother. It's not about a meth-addict mother," he said. "That mother has her own responsibility and her own demons to deal with for the fact that her actions put her daughter in the place where someone had the opportunity to take advantage of her."

The Daily Times is not naming the mother because that would identify the child.

Stephen Taylor, Martinez's attorney, said there will not be enough evidence for a conviction.

In an interview with sheriff's investigators prior to his arrest, Martinez said he was sitting on the couch with the girl. If he touched the girl, it was by accident, O'Brien said.

Taylor said the girl's mother did not report anything to law officers. But four days later, one of the girl's relatives notified the San Juan County Sheriff's Office of the alleged molestation.

He said the girl had been molested by another man in a separate incident while the mother was watching her.

Taylor said he will call an expert witness who has analyzed the girl's sexual-assault exam. He said the witness will say there is not enough physical evidence to say for certain whether the girl was molested.

"There are a lot of issues in this case, and I expect you are going to have a very difficult time," Taylor told the jury. "I don't think the state knows more than anyone else."

The trial is scheduled to last two days. It is being tried in front of District Judge William Birdsall.

The jury is made up of seven men and six women, including an alternate.