The George Washington Carver High School basketball team had built a huge early lead in its playoff game in New Orleans on Saturday when John Shallerhorn tried to walk into the gym. But Shallerhorn, 35, wasn’t wearing a mask, so a staffer blocked his way, police said.

When Shallerhorn punched the staffer, Tulane University police officer Martinus Mitchum, who was working security for the team, rushed to help. That’s when Shallerhorn pulled a gun, police said, and fatally shot Mitchum, 38, in the chest, sending players and fans scurrying for safety.

Shallerhorn, who was quickly arrested and charged with multiple felonies including murder of a police officer, had also robbed someone outside the game before coming inside, according to a criminal complaint reviewed by nola.com.

He confessed to the killing, police said, and was ordered held without bail. It’s not clear if he has an attorney.

The killing stunned New Orleans police and officials and students at Tulane and Carver High, with Mitchum’s colleagues, friends and family remembering him as a hero.

“I feel like Mitch saved lives last night because we don’t know what that gentleman really was coming in there to do,” Jason Bertrand, a friend, told WJTV. “There was kids in there, families in there. What could you be going into a high basketball with a gun?”

The fatal shooting is the latest deadly incident over mask mandates meant to slow the spread of the coronavirus, including several that have targeted police and guards trying to enforce the policies. Last May, a Family Dollar security guard in Michigan was killed after ordering customers to mask up, and in August, a Pennsylvania man was charged with firing an AK-47 at police after arguing about a mask policy at a cigar shop.

Mitchum, a Detroit native who contemplated becoming a preacher before going into law enforcement, made a deep impact at the high schools where he worked after graduating from the police academy in Slidell, La., nola.com reported.

At L.B. Landry High School, where he went by “Mitch,” he became something of a manager and mentor for the basketball team, booking hotels and food and traveling with the squad as it won a state championship. “We probably wouldn’t have those championships we have in basketball if wasn’t for Mitch,” Tarance Davis, the school’s former athletic director, told nola.com.

In recent years, Mitchum, who also worked as a reserve constable for the city courts, adopted a similar position with the Carver team. He was a “fixture” at the team’s games, where he frequently worked security, school officials said.

On Saturday, he was on duty in uniform outside the school’s home gym in New Orleans’s Desire neighborhood as the Carver Rams hosted Warren Easton High School in the first round of the playoffs.

As the teams played, Shallerhorn walked up to an unidentified man in the parking lot, flashed a gun and demanded his jewelry, according to court documents reviewed by nola.com. When he then tried to walk into the gym, he ran into a staffer checking fans’ temperatures at the door and making sure they had masks.

He punched the staffer in the face after refusing to put a mask on, nola.com reported, and then shot Mitchum when he came to help. Other deputies on the scene quickly arrested Shallerhorn, the New Orleans Police Department said in a statement.

Inside the gym, Carver had just scored a layup to go up 22-2 when the gunfire rang out and the game was immediately halted.

Tributes flowed in for Mitchum this weekend. Tulane University called him a “dedicated police professional who had a heart of service.”

“They killed somebody that helped so many African American students come out of there and become something,” Lyn Clark, a former high school football player at a school where he worked, told nola.com. “How do you take the life of someone who helped the lives of so many people who they said weren’t going to be anything?”

On Sunday, Carver’s basketball players wrestled with their own grief as they traveled to another school’s gym to resume their playoff game that was abruptly halted when Mitchum was shot.

“He always had a good spirit,” Solomon Washington, a Carver player, told nola.com after the game. “He was fun to be around. It’s tragic that it happened to him.”