(ABC News) -- Former Minnesota State linebacker Isaac Kolstad allegedly threw the first punch before being knocked out and kicked in a fight with Rutgers quarterback Philip Nelson and an unknown man Sunday, authorities said today.
Kolstad suffered serious brain injuries and is "fighting for his life," according to a statement from his father, Blaine Kolstad.
The fight broke out in the early hours of Sunday morning as the men were all leaving bars in the downtown entertainment district of Mankato, Minn., authorities said.
Nelson and Kolstad exchanged angry words over Nelson's girlfriend, leading Kolstad to punch Nelson, but it was a third, unidentified man who knocked Kolstad out, according to details released by prosecutors today.
Nelson, who transferred to Rutgers this spring, was arrested Sunday and charged today with assault, and police are searching for the third man.
Prosecutors announced details about the fight for the first time today, drawing on statements from witnesses at the scene, including Nelson's girlfriend and Kolstad's friend.
Kolstad and a friend identified by police as "S.T." were walking through the entertainment district when Nelson said something to Kolstad about kissing Nelson's girlfriend. The two got into a heated verbal exchange and Kolstad then allegedly punched Nelson in the back then turned to walk away while S.T. tried to talk to Nelson to de-escalate the situation, prosecutors said.
A third man wearing a red shirt and jeans then came and allegedly knocked Kolstad to the ground. Nelson then allegedly kicked Kolstad in the head twice before turning and leaving the scene with his girlfriend.
Police found Kolstad lying in the street, not moving, and called an ambulance.
One officer tracked down Nelson and made note of his "bloodshot eyes and an odor of alcohol coming from him." Police arrested Nelson and today charged him with two counts of assault.
Nelson told police that he and his girlfriend had been drinking at a bar called Blue Bricks in Mankato and at one point the bouncer tried to kiss his girlfriend, prosecutors said. He did not remember kicking Kolstad and did not know the man in the red shirt.
Nelson's girlfriend, identified as M.V., told police that Nelson and Kolstad were arguing and "some guy came flying in and knocked out" Kalstad.
Police showed images of the man in the red shirt today and are trying to find out who he is and to locate him.
Kolstad is being treated at the Mayo Clinic Health System in Mankato. He spent two seasons playing football for North Dakota State in 2008 and 2009 before transferring to Minnesota State for his junior and senior years in 2011 and 2012, according to ESPN.
Nelson played two years at the University of Minnesota before transferring this year to Rutgers.
Kolstad graduated from Mankato East High School while Nelson graduated from Mankato West High School, according to ESPN.
Nelson is scheduled to be arraigned later today.