The boy's mother reportedly filed a report with Texas Child Protective Services. The results of any investigation are unclear and no charges have been filed in the case.
In a text to NFL Media Insider Ian Rapoport, Peterson's lawyer Rusty Hardin wrote, "The allegations were made over a year ago to CPS and law enforcement and no action was taken. Plus there was an adult witness present in the car and she flatly denied that Adrian caused any injury to the boy."
Hardin also released a statement: "The allegation of another investigation into Adrian Peterson is simply not true. This is not a new allegation, it's one that is unsubstantiated and was shopped around to authorities in two states over a year ago and nothing came of it. An adult witness adamantly insists Adrian did nothing inappropriate with his son. There is no ongoing or new investigation."
Later Monday night, the Vikings released a statement of their own: "As part of the information we have gathered throughout this process, we were made aware of an allegation from 2013 in which authorities took no action against Adrian. We will defer any further questions to Adrian's attorney Rusty Hardin."
Peterson was indicted Friday on a charge of reckless or negligent injury to a child -- a different 4-year-old son -- and faces up to two years in prison and a $10,000 fine if convicted. He flew to Texas and turned himself in early Saturday morning to the Montgomery County (Texas) Sheriff's Office and was released after posting a $15,000 bond.
Peterson's arraignment in Texas was scheduled for Wednesday but was delayed to Oct. 8 as Hardin is out of the country.