Police say the incident that ended with Rutgers quarterback Philip Nelson allegedly kicking an unconscious man in the head as he lay in a street happened because Nelson was upset over a bouncer kissing his girlfriend's hand on Saturday night.

In a criminal complaint obtained by NJ.com, police detailed the events that left Isaac Kolstad in critical condition at the Mayo Clinic Health System in Mankato, Minn., and a neurosurgeon interviewed by police said he is not sure Kolstad will survive.

Nelson was upset because a bouncer at a bar in downtown Mankato flirted with his girlfriend, who told police Nelson is very protective of her. Nelson's girlfriend said they left the bar and encountered Kolstad outside around 2 a.m. on Sunday morning. The bouncer and Kolstad are different people.

Nelson and Kolstad, a former Minnesota State-Mankato football player, had a "heated" verbal exchange and were separated, according to witnesses. A police officer that viewed surveillance footage reported that Kolstad threw a punch that hit Nelson in the back, which forced Nelson to fall to the ground.

Kolstad began to move away and an unknown male ran after him. The unknown male delivered a punch to Kolstad's head/face. Kolstad immediately went limp and collapsed to the pavement. Witnesses told the Minneapolis Star-Tribune that Kolstad was out on his feet after the first punch, then fell back and smacked his head on the pavement before Nelson allegedly kicked him.

Nelson's girlfriend was observed trying to hold him back, but he pushed past and approached Kolstad. Nelson then delivered at least one kick to the left side of Kolstad's head. The police officer said the video clearly shows that Kolstad was defenseless as Nelson delivered the kick(s).

The unknown assailant fled the scene and authorities continue to search for him. Police described him as a white male in his early 20s, wearing jeans and a red T-shirt. Police believe the shirt was torn during the attack. Nelson remained in the area and was identified by several witnesses as the person who kicked Kolstad in the head.

Nelson, 20, was observed by a police officer on the scene as having bloodshot eyes and an odor of alcohol coming from him, the complaint said.

At the Blue Earth County Jail, Nelson was interviewed by a police officer about the incident. Nelson told police that he thought he had been punched by the bouncer who had hit on his girlfriend earlier in the night.

Nelson said he saw the unknown suspect punch Kolstad. Nelson said he didn't know the other suspect. Nelson said he walked away after that. The police officer asked Nelson if he remembered kicking Kolstad in the head, and Nelson replied that he did not, police said.

According to the complaint, Kolstad suffered severe injuries consisting of bleeding of the brain, skull fracture, brain swelling, brain shifting, and deterioration of lungs from lack of oxygen. Kolstad's skull fracture was on the left side of his head, which is the same area Nelson kicked him, police said.

The next day, police spoke to Dr. Dominic Cannella, a neurosurgeon at the Mayo Clinic and asked what could have caused the injury. Police said Cannella said a combination of being struck and kicked in the head. The doctor said Kolstad has a severe head injury and a severe pulmonary injury. The doctor isn't sure if Kolstad will survive and isn't optimistic he'll have a good recovery if he does survive. The doctor said they are taking the situation hour-by-hour.

Nelson has been charged with one count of first-degree assault and one count of third-degree assault, both felonies in Minnesota. First degree assault indicates the perpetrator inflicted "great bodily harm" and carries a maximum prison sentence of 20 years.

Nelson, handcuffed and in an orange jailhouse jumpsuit, appeared in court on Monday afternoon. Bail was set at $20,000 by Blue Earth County District Judge Bradley Walker. Nelson's lawyer, James Fleming, told NJ.com that he expects Nelson to make bail Monday evening.

Fleming said that Nelson “was not the aggressor," according to the Star-Tribune. Fleming reportedly will conduct his own investigation to determine who’s blow caused the injuries to Kolstad.

Nelson remains on the Rutgers roster.

"We are in the process of gathering information on the situation and reserve comment until the legal process is complete," Rutgers athletic department spokesman Jason Baum said in a statement.