Initial reports said the boy was in a hospital in critical condition after the incident involving a 27-year-old man who is in a relationship with the mother. The man is being held on multiple charges and $750,000 bail.
Earlier Friday, Peterson was at the Vikings' practice facility and requested not to be asked about the incident. His father, Nelson, confirmed to the St. Paul Pioneer Press that the victim was his grandson.
"I want to start off by saying I really appreciate all the support that I’ve been receiving from fans, the Vikings organization," Peterson said. "This is a private matter and I would ask y’all to please just respect my privacy and not ask at all about the situation at hand."
Despite the tragic news, Peterson said he was determined to play Sunday when the Vikings host the Carolina Panthers.
"I plan on playing Sunday. I will be playing Sunday, let me correct that. Be ready to roll, focused and ready to get a 'W' on Sunday, being 1-0."
The exact identity of the son remained a mystery Friday evening, and police say they will wait until next week to reveal a name. Peterson has a son — Adrian Jr. — who an early TMZ report said was the victim, but later reports — and Peterson's father — denied that.
Other reports said the child involved in this incident was not publicly known to be Peterson's, and some suggested Peterson wasn't even sure the child was his, though Nelson Peterson did tell the Pioneer Press that the victim was one of Adrian's sons.
After news of the death, Peterson took to Twitter to thank well-wishers, though he did not confirm the boy's identity:
Thank you to my family, my fans and fans of other teams for their support.
Joseph Robert Patterson has been charged with aggravated assault and aggravated battery of an infant against his girlfriend's son following the Wednesday incident, Lincoln County State's Attorney Tom Wollman said in a briefing Friday. Authorities said they will review the case for further charges in light of the death.
Patterson called police just before 6 p.m. local time Wednesday reporting that the child had choking injuries, according to the Argus Leader in Sioux Falls.
Sioux Falls Police Lt. Blaine Larsen said the child was taken by ambulance to a nearby hospital and said it became clear the injuries appeared to be "consistent with abuse" and not accidental.
Peterson was back with the team after missing Thursday's practice. He was on the field for stretching with the Vikings on Friday morning. He left for the locker room briefly during special teams drills, which he doesn't participate in, and returned later.
''Football is something I will always fall back on,'' Peterson said. ''It gets me through tough times. Just being around the guys in here, that's what I need.''
Vikings head coach Leslie Frazier said that even though Peterson had vowed to play, the team would play it by ear in such a delicate situation.
"He has our prayers and support from this football team, from this organization as he’s dealing with a personal matter," Frazier said, according to the Minneapolis Star-Tribune. "We’ll see how things go with him. We expect him to play, but this is a very personal situation that he’s dealing with. We’ll talk to him in the next 24 hours."
Fellow Vikings running back Toby Gerhart said: ''It's hard for any man to admit that he's hurting or he needs help or anything like that. For us to be around him and tell him we've got his back, if there's anything he needs that we're there for him, I think that goes a long way.''
Peterson is second in the NFL with 421 yards rushing and first in the league with five touchdowns.