ST. PETERSBURG, Fla. (WFLA) – The man accused of punching a University of South Florida student and causing severe brain trauma was arrested Thursday afternoon.

Police said Zach Allen, 23, punched Brian Neal on St. Patrick’s Day on Central Ave, next to Detroit Liquor store.

Security camera video shows three men strolling the sidewalk along Central Avenue in St. Petersburg, early St. Patrick’s day morning.

In front, according to St. Petersburg Police, is Allen.

Allen is a student of Jiu Jitsu and trains at a studio in Largo. His instructor cannot believe what he’s hearing.

“I could not see Zack ever doing something like this. He’s been training with me since he’s 15. He’s a family guy,” said Rodrigo Greeno.

“I’m shocked that this situation happened” said Greeno, who said the form of martial arts he teaches does not involve punches. “We teach Brazilian Jiu-Jitsu, which is take-downs,” he said.

Detectives learned Neal exchanged words with Allen over being inappropriate.

“He was cat calling and whistling to females in the general area. The victim made a comment to him,” said Major Shannon Halstead, with the St. Petersburg Police Department.

Then suddenly, a punch to the face. One blow. Neal fell to the pavement, hitting his head, hard.

Jenny Lucien and her girlfriend came upon the scene. Neal wasn’t moving.

“It was very terrible. Ya know, everybody’s enjoying their time and that guy was probably enjoying his time too. I feel bad for the family,” said Lucien.

Allen didn’t say a word as cops put him in a squad car.

His Jiu-Jitsu instructor insists the Zach he knows isn’t violent.

“Never had any problem. Very well educated kid. Known as smart person, always treats people, everybody with a lot of respect,” said Greeno.

Greeno feels bad for the victim. So does the woman who saw the aftermath.

“Condolences for his family and I hope everything works out,” said Lucien.

The search for his attacker went on for five days. Police said thanks to social media and dozens of tips, there was a break in the case.

“As you saw, it was shared several times on Facebook and we had a lot of information come in about the identity on all the different parties that we’re involved,” said Sgt. Halstead. “I think the information that we received from the public in this case was instrumental in bringing it to a close.”

Detectives said the two men who were with Allen the night of the altercation are witnesses.

Allen’s attorney, Marc Plotnick, asked that everyone wait to judge his client until all the evidence is presented.

Neal remains at Bayfront Health in critical condition and will require a long-term recovery. He may have life-altering brain injuries.

Allen is expected in court Friday for first appearance.