(CNN) Cell phone data, shell casings and a gun helped police link a 24-year-old man to a series of fatal shootings that paralyzed Tampa residents for nearly two months, investigators said.

Howell Donaldson III faces four counts of first-degree murder in the killings of three men and a woman over roughly five weeks in October and November in Tampa's Seminole Heights neighborhood.

Still, the motive behind the killings remains a mystery.

Donaldson was arrested late Tuesday and appeared in court Thursday morning for his first hearing. In court, he stood quietly in handcuffs wearing a green safety smock designed to prevent self-harm. He was ordered held without bond until a hearing Tuesday. He was appointed an attorney and filed for indigent status.

Donaldson has not confessed to the killings, police have said, adding that investigators have more work to do.

"He was friendly and nice to the cops, but he didn't give us anything," Tampa Police Chief Brian Dugan said of Donaldson's conversation with police after his arrest. "He didn't tell us why he was doing it or anything like that."

During interrogations, Donaldson admitted to owning the weapon, police said. He said he was unfamiliar with the Seminole Heights neighborhood and did not have an association with anyone in the area, an arrest affidavit states.

His firearm and ammunition matched the weapon police believe was used for the four murders, and his cell phone location data connected him to an address near the scenes of the first three killings, according to the affidavit. Presented with that evidence, Donaldson declined to explain and requested the presence of an attorney, the affidavit states.

Donaldson has not yet been charged with four counts of murder because capital offenses in Florida require a grand jury indictment, State Attorney for Hillsborough County Andrew Warren said Thursday. Prosecutors have 21 days from the date of arrest to bring those charges, he said.

Death penalty possible

Warren said he would consider aggravating and mitigating factors in deciding whether to seek the death penalty against Donaldson.

"The death penalty should be reserved for the worst of the worst offenders in our society, and general speaking, a serial killer would qualify, but it is my duty to follow the law," he said. "If the case satisfies the legal requirements and if it's consistent with what the victims' families want, then we will seek the death penalty."

At least one of the victims' relatives said she would back that move.

Maria Rodriguez, the stepmother of Anthony Naiboa -- one of the four people killed in the shootings -- told CNN she wanted the perpetrator to get the death penalty.

For his part, Tampa Mayor Bob Buckhorn said Wednesday the death penalty would be an appropriate punishment.

"I think at the end of this, if he is found to be guilty, he should die," Buckhorn said. "It's that simple."

This high-profile case comes in the midst of an ongoing debate in Florida over prosecutors' discretion on capital punishment.

Warren, who was was elected last year, said he had sought the death penalty in three cases since taking office.

A 'quiet, polite young man'

Donaldson or "Trai," as many call him, did not stand out as a violent person for many who knew him.

"He is the most quiet, polite young man," said Tony Estevez, a neighbor who has known Donaldson for about 20 years. "I've never seen him say a bad word."

The 24-year-old attended Alonso High School in Tampa for half of his junior year and his senior year, the Hillsborough County School District confirmed.

From there, he attended St. John's University in New York beginning in fall 2011, and he graduated in January 2017, according to Brian Browne, executive director of university relations.

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He was a walk-on for the men's basketball team during the 2011-2012 season but never played in a game, Browne said.

His freshman-year suitemate at St. John's -- who asked to not be identified -- said Donaldson had the best manners, dressed very well, and was a sneaker head.

The former suitemate said he never saw Donaldson angry, and at parties, he was a wallflower.

"[Trai was] just a regular guy," the former suitemate said. "Nothing stood out as violent."

Donaldson graduated with a bachelor's degree in sports management, CNN affiliate WFTS reported.

After college, Donaldson appeared to have returned to Tampa earlier this year. He worked at Ultimate Medical Academy, a health workers training school, for a few months until he was terminated for absenteeism, according to WFTS.

Most recently, he worked at a Tampa-area McDonald's that is about four miles from the Seminole Heights neighborhood.

Following his arrest, Donaldson was "cooperative" and seemed perfectly "fine" during police interviews, Dugan said.

"He seemed like he knew exactly what he was doing and what was going on. He was very much aware of where he was and what he was doing," the police chief said.

'We are relieved to a point'

Residents in the tree-lined neighborhood of Seminole Heights are no longer living with fear.

For more than a month, families stopped taking long walks or going on morning runs and kept their lights on at night. The neighborhood was swarmed by officers in patrol cars for weeks and lately, mounted patrols and police helicopters also became a familiar sight.

After learning that Donaldson had been arrested, Lajuanda Barrera walked to the front door of her flower shop and took down a poster that offered a reward for information about the killer.

"We are simply overjoyed. I think that's the word. Relief, utter and utter gratitude to all the men and women that are responsible to help bring this monster to justice," Barrera told CNN affiliate WFLA

Rodriguez, the stepmother of Naiboa, told CNN she was happy no more people will be harmed.

"We're relieved, to a point," she said. "Justice has been served, so far."