(CNN) -- A mother who initially told police that her son was abducted in Florida admitted during an interrogation that she led him to a canal -- where the 9-year-old was later found dead, an arrest affidavit says.

Patricia Ripley, 45, initially told Miami-Dade Police that two men side-swiped her car while she was driving with her son, Alejandro Ripley, around 9 p.m. Thursday, officials said.

She said the men demanded drugs and abducted her son, who has autism and is nonverbal, authorities said.

Contradicting witness statements and video footage later surfaced, and she admitted to her son's death.

"She stated he's going to be in a better place," the Miami-Dade County Police said in the affidavit.

Ripley was arrested and charged with with first-degree murder and first-degree premeditated attempted murder, according to the affidavit.

"At this point the contents of the arrest affidavit are merely allegations and not proof of Mrs. Ripley's guilt ... any conclusion about the case is grossly premature," defense attorney Nelson Rodriguez Varela told CNN in a statement.

"Our position is that Mrs. Ripley is a good woman and mother who always looked after the best interest of her son Alejandro. This is not the time to rush to judgment in this matter. At this very preliminary stage, Mrs. Ripley is presumed innocent as any person charged with a crime would be."

Video shows boy was pushed into a canal

Ripley initially told police the driver of the vehicle had a knife, got out and demanded drugs.

When she didn't provide drugs, the man opened the car door and took her cellphone and tablet, she said, according to the affidavit. Then he kidnapped Alejandro, it added.

The boy's body was found hours later still wearing a Captain America T-shirt, said Katherine Fernandez Rundle, the Miami-Dade State Attorney.

Once she was taken to the missing person's office, police said, Ripley began providing conflicting statements about what took place.

Ripley was then taken to the homicide bureau where she was read her Miranda rights.

"These statements ... contradicted the statements of witnesses and the video footage obtained from the area," police said in the affidavit. "Upon being confronted with this information, the defendant recanted her initial story including that she had been robbed."

Video footage shows Ripley pushing the victim into a canal around 7:30 p.m. Thursday. Nearby residents heard screaming, found Alejandro in the canal and rescued him, Rundle said.

Ripley took Alejandro to a different canal an hour later where the boy drowned, the affidavit said.

She was arrested and booked in the Turner Guilford Knight Correctional Center on Saturday, according to Miami-Dade County Corrections records.

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