TAMPA, Fla. -- A Michigan woman has claimed detectives used her to force her son to confess to killing a 13-year-old Ruskin girl in April, a newspaper reported Saturday.

Refugia Whitten met her son David Onstott April 16 at the Hillsborough County Sheriff's Office shortly before he reportedly confessed to killing Sarah Lunde, according to The Tampa Tribune.

Whitten said Onstott told her that detectives would not let him see her unless he confessed.

According to court records, after the tearful conversation with his mother and a brief phone call to his attorney, Onstott told detectives he choked Sarah and said: "I did it. I'm guilty of murder."

An autopsy found that Sarah died from crushing blows to the head.

"He signed the statement to make it end," Whitten, who lives in Michigan, said in telephone interviews.

A message left Saturday evening for sheriff's officials was not immediately returned. The public defender's office has said it is looking into Onstott's statement.

Onstott, a 37-year-old registered sex offender, has pleaded not guilty to charges of first-degree murder and attempted sexual battery.

Whitten told the newspaper that she believes Onstott is innocent.

"He never confessed to me once, and I never heard him confess to anyone else," she said. "I regret ever saying anything to him. I regret telling him to just say the truth because they used me to make it look like he confessed to me. They used me to break him down."

Prosecutors on Thursday released several hours of recorded telephone conversations Onstott made from jail. In one call to his ex-wife, he said: "You know, I've broken every commandment now. Every single one." In other calls, Onstott consoles his mother and offers her financial advice.

Sarah's partially clothed body was found April 16 in an abandoned fish pond near her family's mobile home. She was last seen April 9 shortly after returning to her Ruskin home from a church trip, but not reported missing until two days later.

Prosecutors plan to seek the death penalty for Onstott. His next court hearing is scheduled for Feb. 27.

Ruskin is about 15 miles south of Tampa.