Story highlights The University of South Alabama says the student was acting erratically

The school says the student charged the officer, who drew his gun

18-year-old Gilbert Thomas Collar was killed in the Saturday shooting

His mother and friend says he was a varsity wrestler and a good-natured guy

Authorities are investigating why a University of South Alabama officer fatally shot an 18-year-old f reshman who they say was naked and acting erratically outside the campus police station early Saturday.

With few details of the shooting in the Gulf Coast city of Mobile, the student's mother and one of his friends said they could not understand how a six-year varsity wrestler and good-natured teenager could have died under such strange and sad circumstances.

According to a statement from the school, the campus police officer heard a loud banging noise on a window at the station at 1:23 a.m. CT (2:23 a.m. ET) Saturday. When he left the station to investigate, the school said, "he was confronted by a muscular, nude man who was acting erratically."

The man, later identified as Gilbert Thomas Collar, of Wetumpka, Alabama, repeatedly rushed and verbally challenged the officer in a fighting stance, the school said.

The officer, whose name hasn't been released, drew his weapon and ordered Collar to stop, the school said. The officer retreated several times to try to calm the situation.

"When the individual continued to rush toward the officer in a threatening manner and ignored the officer's repeated commands to stop, the officer fired one shot with his police sidearm, which struck the chest of the assailant," the school statement said. "The individual fell to the ground, but he got up once more and continued to challenge the officer further before collapsing and expiring."

Collar's mother, Bonnie, said the two people who called her with the news of her son -- someone from the school and another involved in the investigation -- did not mention that her son was trying to attack anyone when he was shot.

"He was wearing no clothes and he was obviously not in his right mind," she told CNN. "No one said that he had attacked anybody, and obviously he was not armed. He was completely naked."

Bonnie Collar said she did not know why her son was acting that way when he was killed. She said he weighed 135 pounds and was 5-foot-7 with a wrestler's build.

"The first thing on my mind is, freshman kids do stupid things, and campus police should be equipped to handle activity like that without having to use lethal force," she said.

Campus police immediately contacted the district attorney's office to request an external investigation, and the Mobile County Sheriff's Department will assist, the school said.

The officer has been placed on paid administrative leave pending the outcome of internal and external investigations, according to the school, which enrolls about 15,000 students.

Investigators are looking at security camera tape of the shooting, Collar's mother said. CNN's calls about the tape were referred to school spokesman Keith Ayers, who did not immediately return a message seeking comment.

Earlier Saturday, Ayers called it a "campus tragedy" for the university family but offered no other details, citing the active investigation.

One of Collar's oldest friends was Chris Estes, 18. He said the boys became friends at age 5 and grew up playing baseball together. Along with their friend Jared, they became three best friends with the slogan "JGC for life," Estes told CNN by e-mail.

"Gil was a very 'chill' guy, mellow and easy going," Estes wrote. "That's why I don't understand the story that he attacked the cop. He got along with anybody at school no matter who you were. He could always have a conversation with anyone. As many times as I've hung out with Gil, I've never seen aggression in him, especially not towards a cop."

As the boys grew up in Wetumpka, Estes said, Collar stopped playing baseball to focus more on his dominant sport, wrestling. Collar's mother said he was a two-time state qualifier in wrestling, and Estes said he could have wrestled at the collegiate level if he chose.

"Gil loved to hang out with friends, he loved having a good time and made the best out of every situation, always keeping his head up," said Estes, who stayed behind to attend nearby Auburn University at Montgomery.

Estes said it's unfortunate he didn't go off to college with his friend. "If I did, I think the whole situation would have been avoided," he said.

Collar's mother said their hometown of nearly 8,000 people is in disbelief about the shooting. On Twitter, some used the hashtag #WetuFam (Wetumpka Family) on Saturday in remembering Collar.

"Our entire community is in shock because this is so different than his demeanor and his personality that we've seen for the 18 years that he's been on this earth," she said.