How a McDonald's food bag led Tampa police to alleged serial killer The suspect was arrested Tuesday after handing over a gun at a McDonald's.

 -- A McDonald's bag that Howell Donaldson III handed to his colleague at the fast-food restaurant where he worked in Tampa was the key piece of evidence that led police to the alleged serial killer, authorities said today.

A person identified as "Witness 3" approached a police officer at the McDonald's restaurant in Tampa's Ybor City neighborhood Tuesday afternoon to tell them that an employee, Donaldson, gave them a "McDonald's food bag" and said he planned to leave the state, according to a criminal report affidavit filed early today in Florida's Hillsborough County.

Inside the bag was a .40-caliber Glock firearm loaded with SIG brand Smith and Wesson ammunition -- the same weapon used in a string of mysterious slayings over the past six weeks in Tampa's Seminole Heights neighborhood, officials said.

That gun was what investigators needed to identify the alleged killer, Tampa Police Chief Brian Dugan told reporters at a news conference this afternoon.

Donaldson, 24, allegedly purchased the legal firearm six days before the first killing on Oct. 9, authorities said. He picked up the handgun and bought a 20-round box of SIG brand Smith and Wesson ammunition on Oct. 7, according to the affidavit.

The Florida Department of Law Enforcement and the Bureau of Alcohol, Tobacco, Firearms and Explosives later discovered cartridge casings at all four crime scenes that matched the .40-caliber Glock possessed by the suspect, the affidavit states.

Moreover, call detail records showed that Donaldson's cellphone was geographically associated with a cell tower near the locations of the first three slayings in the Seminole Heights neighborhood within minutes of their taking place, according to the affidavit.

Donaldson, also known as Trai, was detained at the McDonald's in Ybor City on Tuesday afternoon. He will be charged with four counts of first-degree murder in connection to the killings. He is being held in Hillsborough County Jail in the meantime and has not yet been assigned an attorney, according to Dugan.

The first three victims were killed within an 11-day span in October. Benjamin Mitchell, 22, was killed Oct. 9, and the body of 32-year-old Monica Hoffa was found Oct. 13, half a mile from where Mitchell was killed. Anthony Naiboa, a 20-year-old man authorities described as having mild autism, was killed Oct. 19 after taking the wrong bus home from work, according to police.

The fourth victim, 60-year-old Robert Felton, was fatally shot from behind in the early hours of Nov. 14.

"To have four homicides on your watch is a tough pill to swallow," Dugan said at this afternoon's press conference. "I will hold onto that the rest of my life."

The Tampa police chief said the individual who turned in the weapon to authorities is a female manager at the McDonald's in Ybor City, where Donaldson is a former employee. Dugan said he doesn't believe Donaldson gave his coworker the bag so that he would get caught, but rather thought she would hold onto it for safekeeping without looking inside.

Delonda Walker, the McDonald's general manager, said in a statement that she is "overwhelmed and surprised by yesterday's events," but emphasized that she will be speaking exclusively to police.

Walker is "appreciative" that police were nearby and "acted quickly," she said.

"I am also appreciative of the outpouring of support from the community," Walker said. "My thoughts are focused on the victims and their families and out of respect for them and the active investigation, please direct inquiries to the Tampa Police Department."

Earlier today, in an interview on ABC News' "Good Morning America," Dugan said he spoke to the suspect briefly after police interviews Tuesday night and that the suspect was "pretty nonresponsive." Donaldson has not admitted to committing the killings but told police he is the owner of the .40-caliber Glock, according to Dugan.

"He wasn't angry. It was a very subdued, strange behavior, in my opinion," Dugan said.

The investigation is ongoing and there are "so many still unanswered questions," the police chief added. Investigators are unsure of a motive behind the four killings and do not yet know of a connection between Donaldson and the Seminole Heights neighborhood where the slayings took place. Police do not believe there are any other suspects in the case.

Nevertheless, Tampa Mayor Bob Buckhorn said residents in the Seminole Heights neighborhood traumatized by the recent killings "can rest comfortably" now that the suspected serial killer has been arrested.

"This was a good night last night, thanks to the work of the Tampa Police Department and all of our partners and Chief Dugan," Buckhorn said in an interview this morning on "GMA."

"The folks in Seminole Heights, we think, can rest comfortably that this individual is in custody, and now the process starts moving forward. But it's a good day in Tampa."

Donaldson was a graduate of St. John's University in New York City and a Tampa native.

"We can confirm that Howell Donaldson III attended the university beginning in the fall of 2011 and graduated in January 2017," St. John's University spokesman Brian Browne told ABC News. "He was a walk-on student-athlete for the men’s basketball team during the 2011–2012 season but never played in a game."

Donaldson was arrested in Manhattan in 2014 but the case is sealed, the New York City Police Department said.

The Tampa Police Department received more than 5,000 tips related to the string of slayings that left the Seminole Heights neighborhood on edge. The police chief said law enforcement presence in the neighborhood will begin scaling back following Donaldson's arrest.

"You know, it's been 51 days since the first murder, and it's day one of the healing process to kind of help these families and this community put their lives back together," Dugan told ABC News.

In a statement released late Tuesday, Florida Gov. Rick Scott commended the "relentless effort" of Tampa police, who worked through the Thanksgiving weekend on the case.

"As a father and a grandfather, it is unimaginable what the families of these victims are going through," Scott said. "Our thoughts and prayers remain with them, and we will work to ensure that they receive the justice they deserve. In Florida, we have absolutely zero tolerance for this type of evil behavior, and anyone responsible will be held to the fullest extent of the law."

ABC News' Mark Crudele, Aaron Katersky and Rex Sakamoto contributed to this report.