An alleged serial killer accused of terrorising a Florida suburb and shooting dead four people in little over a month, was caught after he asked a colleague to hold a bag containing a loaded gun, authorities have revealed.

The break in the case came on Tuesday when Howell Emanuel Donaldson, 24, brought the fast-food bag containing the loaded Glock to his job at a Tampa McDonald's and asked a co-worker to hold it while he went across the street.

The colleague was rattled and when Donaldson left, handed the bag to the manager, who handed it to a police officer who was doing paperwork at the restaurant.

Florida Governor Rick Scott hugs Tampa police officer Randi Whitney, the officer credited with the arrest of Howell Emanuel Donaldson. Photo / AP
Florida Governor Rick Scott hugs Tampa police officer Randi Whitney, the officer credited with the arrest of Howell Emanuel Donaldson. Photo / AP

The cop, identified as Officer Randi Whitney, called for backup and by the time Donaldson had returned from his errand, police were waiting.

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"The gun is what we needed," Tampa police chief Brian Dugan said at a news conference surrounded by relatives of the victims.

The fast food worker told the officer a colleague, Donaldson, handed over a McDonald's food bag and said he planned to leave the state, according to a criminal report affidavit filed in Florida's Hillsborough County, ABC News reported.

Inside the bag was a .40-calibre 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 the suburb of Seminole Heights, officials said.

The arrest overnight brought relief to a community terrified about when the killer would strike next.

The first shooting was on October 9, followed by two more shooting deaths on October 11 and October 19. By Halloween, the fear was so great that police escorted children while trick-or-treating. The fourth killing happened on November 14.

"We had a community that was on edge," Mayor Bob Buckhorn said. "Today the light shines. The darkness is over. This community begins the healing process."

Donaldson did not live in Seminole Heights where the shootings happened and told investigators he was unfamiliar with it, AP reported.

Howell Donaldson has been charged with four murders.
Howell Donaldson has been charged with four murders.

He graduated from St. Johns University in New York in January, according to university spokesman Brian Browne. He was a walk-on for the men's basketball team during the 2011-12 season, but never played in a game, Browne said.

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After graduating, he worked in customer support at the Ultimate Medical Academy, a school that trains workers for health care jobs. He started on February 13, but was fired after about three months for absenteeism, according to an academy spokesman.

His LinkedIn account also listed a job as a "guest experience host" for the New York Mets last year. The Mets would neither confirm nor deny he worked for them when approached by AP for comment.

Police in New York said Donaldson had been arrested in May 2014, but the arrest was sealed and no details were available.

He was employed as crew chief at the Ybor City McDonald's at the time of his arrest.
"The person who called us, I cannot thank them enough for standing up and doing the right thing," Dugan said.

Authorities said Donaldson bought the gun and a 20-round box of bullets from Shooter's World in Tampa on October 3. He picked it up after the four-day waiting period and the first killing happened two days after that.

Detective Austin Hill wrote in a police report that Donaldson told investigators "no one, except for himself had control of the Glock firearm since his purchase".

Analysis of Donaldson's mobile phone found location data that indicated three days of recorded times and activities corresponding with the first three shootings.

The arrest report said police found clothes in Donaldson's car that were similar to those worn by a person spotted in a surveillance video taken on the night of the first shooting.

The police chief said authorities do not know why Donaldson chose the Seminole Heights neighbourhood.

Residents and police had been on edge since October 9, when Benjamin Mitchell, 22, was shot to death.

On October 11, Monica Hoffa, 32, was slain. And on October 19, Anthony Naiboa, 20, was killed after taking the wrong bus home from his new job. The fourth and final shooting took place on November 14, claiming the life of Ronald Felton, 60.

All of the October victims were either getting on or off a city bus, or were at a bus stop when they were shot, police said.