CORRECTS FIRST NAME TO ROBERT, NOT JOHN - This photo provided by the City of Biloxi, Miss., Public Affairs Department shows police officer Robert McKeithen who was killed Sunday night, May, 5, outside the department building in Biloxi. (City of Biloxi, Miss., Public Affairs via AP)

BILOXI, Miss. (AP) — Residents in a Mississippi Gulf Coast city continued on Tuesday to mourn the death of a longtime officer who authorities said was gunned down by a man whose mental health had, according to his mother, been sliding in recent months.

A local NAACP leader said the man had threatened to shoot his goddaughter in 2018 when both were high school students.

A public memorial service was held Tuesday evening for Biloxi Patrolman Robert McKeithen, who was shot to death in the parking lot outside the city's police station Sunday night. Darian Tawan Atkinson, 19, is jailed without bail on capital murder charges. McKeithen, a Biloxi officer for 23 years, was shot multiple times. It remains unclear whether the 58-year-old was targeted or shot at random.

Gulfport Police spokesman Lt. Joshua Bromen said search warrants were being served Tuesday. He declined to say whether a gun had been found.

Atkinson's mother, Pamela Broger, told The Associated Press that if her son shot McKeithen, it was because of mental health problems Broger said had developed in recent months.

"Darian went to Texas to work in August," Broger said. "He came back two months later and he was a completely different person. I don't know if something happened there because he wouldn't talk to me. I don't know if it was drugs."

Broger said she had called police officers for assistance, but said officers told her they couldn't do anything because Atkinson wasn't hurting anyone.

"I'm not saying he didn't do it," Broger said. "But if he did, it was because he was suffering."

She also denied that her older son, Davian Atkinson, had done anything to merit being arrested. The 21-year-old was booked into the Harrison County jail early Tuesday on an accessory charge alleging he aided his brother.

Broger said Darian Atkinson came home Sunday night when the family didn't know about the shooting, and left Monday in Davian Atkinson's car. Bromen said "obviously something more is involved" in the brother's charge, noting the mother wasn't charged.

Others see Darian differently than his mother. Biloxi NAACP chapter President James Crowell told the Sun Herald that Darian Atkinson threatened his goddaughter in February 2018 when the two were Biloxi High School students discussing the shooting at Florida's Parkland High School.

"He said, 'I'm thinking about doing the same thing here and I'm going to start with you,' " Crowell said Tuesday.

He said that his goddaughter's family reported the threat to Biloxi High School, but nothing happened until they demanded an investigation. Crowell held a news conference at the time, saying parents should have been notified. WXXV-TV reported then that the accused student, never named publicly, was suspended for three days.

"Even back then, he was threatening people," Crowell said Tuesday. "I guess he went ahead and did it this time rather than just talk about it."

Spokeswoman Jennifer Pyron said privacy laws prevent release of student records. Gulfport Police Chief Leonard Papania, who's leading the investigation, didn't respond to a text from the AP.

At the police station where McKeithen was killed, mourners continued to deposit tributes on his police SUV, overflowing onto tables the city set up.

Patricia Underwood said she drove from Gulfport to pay her respects to a man who convinced her goddaughter to remain in high school. She said her goddaughter called her in tears Monday night.

"She just cried and cried," Underwood said. She said her goddaughter was working at a Biloxi Waffle House two years ago and told McKeithen, a regular at the restaurant, that she was thinking of dropping out.

"He told her she didn't need to get out of school; she needed to stay in and walk across that stage and graduate. And she did," Underwood said.

She was joined by Linda Virgil, who retired from a Dillard's department store where McKeithen did off-duty work as a security guard.

"He would stay later to walk us to our cars every night," she said.

She said he loved to talk about his wife and the diamond rings he bought her, and about his children. "He helped everybody," Virgil said.

Biloxi Police Capt. Milton Houseman said he was one of the officers who brought in Atkinson. Asked if he had any idea why Atkinson smiled as he was brought into the police department after his arrest 40 miles (65 kilometers) away in Wiggins, Houseman said, "I wouldn't venture a guess as to what was in his mind."

Atkinson's first appearance in court could come as early as Wednesday, when a judge will consider bail and could appoint a lawyer. His brother Davian Atkinson is held on $500,000 bail.

State criminal records show no felony convictions for either Atkinson brother. Davian Atkinson was cited for speeding and other traffic offenses once in 2016 and once in 2017.

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Amy reported from Jackson, Mississippi.

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Follow Jeff Amy at http://twitter.com/jeffamy

In this May 2019 security camera image released by the Mississippi Bureau of Investigations, a suspect in the death of a Biloxi police officer is seen. Authorities say the man walked up to the Biloxi officer in the station's parking lot Sunday night, May 5 shot him multiple times and then ran off. (Mississippi Bureau of Investigations via AP)
In this Monday, May 6, 2019 photo provided by the Stone County Enterprise newspaper, Darian Tawan Atkinson, wanted for capital murder in the slaying of Biloxi Patrolman Robert McKeithen, awaits transport back to Biloxi, after being arrested in Wiggins, Miss.  (Jody O'Hara/Stone County Enterprise, via the AP)
This undated photo provided by the Biloxi Police Department via the City of Biloxi, Miss., Public Affairs Department shows Darian Tawan Atkinson. Police on Mississippi's Gulf Coast said late Monday, May 6, 2019, that they have captured Atkinson, a suspect in the slaying of a uniformed officer gunned down outside a Biloxi police station, concluding a tense manhunt. (Biloxi Police Department/City of Biloxi Public Affairs via the AP)
In this photo provided by WLOX, an unidentified Biloxi, Miss., police officer walks away after placing an angel statuette on Biloxi Police Officer Robert McKeithen's official vehicle, Monday, May, 6, 2019. McKeithen was shot in an ambush in front of the police station Sunday night. (John Fitzhugh/WLOX via AP)
Citizens join hands and offer prayer Monday afternoon, May 6, 2019, at the Biloxi, Miss., Police Department, around the official SUV of Officer Robert McKeithen, who was murdered outside the building Sunday night. Area residents decorated the vehicle with memorials, flowers, and messages throughout the day. (Lauren Walck/The Sun Herald, via AP)
The police SUV of police Officer Robert McKeithen is decorated with memorials, flowers, and messages outside the Biloxi Police Department in Biloxi, Miss., Monday, May 6, 2019, a day after the officer was killed outside the building. (Lauren Walck/The Sun Herald, via AP)
Natalie Rickert of Biloxi, Miss., and her daughters Alethea, 9, right,, London, 7, left, and Haven 5, hidden, look at a Biloxi police vehicle that was assigned to patrolman Robert McKeithen that is covered in flowers, Tuesday, May 7, 2019, in Biloxi, Moss. Houseman said he was one of the officers who brought Darian Tawan Atkinson, 19, to Biloxi after his arrest in Wiggins on May 6, in the death of Patrolman McKeithen. (AP Photo/Janet McConnaughey)
A Biloxi police vehicle that was assigned to patrolman Robert McKeithen is covered in flowers as Biloxi Police Capt. Milton Houseman, rear, talks , with a member of the Combat Veterans Motorcycle Association, Tuesday, May 7, 2019, in Biloxi, Moss. Houseman said he was one of the officers who brought Darian Tawan Atkinson, 19, to Biloxi after his arrest in Wiggins on May 6, in the death of Patrolman McKeithen. (AP Photo/Janet McConnaughey)
In this Monday, May 6, 2019 photo provided by the Stone County Enterprise newspaper, a Gulfport, Miss., police officer checks the pockets of Darian Tawan Atkinson, wanted for capital murder in the slaying of Biloxi Patrolman Robert McKeithen, as he is apprehended in Wiggins, Miss. (Jody O'Hara/Stone County Enterprise via the AP)
In this Monday, May 6, 2019 image taken from video, shooting suspect Darian Tawan Atkinson is lead into a police station in Biloxi, Miss. Police on Mississippi's Gulf Coast captured Atkinson, a suspect in the slaying of a uniformed officer gunned down outside a Biloxi police station, concluding a tense manhunt. (The Clarion-Ledger via AP)
Biloxi Police Chief John Miller speaks during a memorial service for slain Patrolman Robert McKeithen while McKeithen's family listens outside the Biloxi Public Safety Center Tuesday, May 7, 2019, in Biloxi, Miss. People gathered Tuesday outside the police station, steps away from where Patrolman McKeithen was gunned down Sunday night in the parking lot. (AP Photo/Janet McConnaughey)
Pamela McKeithen, left, prays as pastors lead a prayer vigil Tuesday, May 7, 2019, in honor of fallen Biloxi police officer Robert McKeithen, who died after being shot multiple times in the parking lot of the Biloxi Police Department, Sunday night. (Justin Mitchell/The Sun Herald via AP)
Dozens of people bowed their heads to honor fallen Biloxi police officer Robert McKeithen during a prayer vigil on Tuesday, May 7, 2019, outside the police department in Biloxi, Miss. A Biloxi police officer, McKeithen died after being shot multiple times in the parking lot of the Biloxi Police Department, Sunday night. (Justin Mitchell/The Sun Herald via AP)