Howell Emanuel Donaldson III
Howell Emanuel Donaldson III with two unidentified women. Facebook

A recent college graduate who has been accused of four slayings that terrorised a Tampa neighbourhood had "no apparent motive" and used the same gun to kill innocent people near bus stops, the police chief said Wednesday.

The crack in the case that has lasted 51 days occurred when the suspect, 24-year-old Howell Emanuel Donaldson, brought a loaded gun to his job at a McDonald's and asked a co-worker to hold it, authorities said.

Workers at the fast-food restaurant reported the gun to a police officer who was doing paperwork there, setting off an investigation that linked Donaldson to the gun used in the crimes. Authorities also said location data from Donaldson's cellphone put him at the scene of at least three of the killings.

"The gun is what we needed," Police Chief Brian Dugan said at a news conference surrounded by family members of the victims.

The arrest overnight (29 November) brought relief to a community on edge over an apparent serial killer who targeted three people near bus stops after dusk last month. Police had earlier released black-and-white surveillance video showing a man in a hoodie as a possible suspect, and by Halloween the fear was so great that police escorted children while trick-or-treating.

The tip that led police to Donaldson came from one of his co-workers at an Ybor City McDonald's, which is near the Seminole Heights neighbourhood.

Donaldson asked an employee at the restaurant to hold a bag with a loaded semi-automatic gun while he went to a nearby business to get a payday loan, according to an arrest report.

The employee told her manager about the gun and the manager alerted a Tampa police officer at a table in the restaurant.

When Donaldson returned to the McDonald's, police were waiting.

"The person who called us, I cannot thank them enough for standing up and doing the right thing," the chief said.

Authorities said a search of Donaldson's cellphone found location data that indicated three days of recorded times and activities corresponding with the first three shootings on 9 October, 11 October and 19 October.

The arrest report said police found clothing inside Donaldson's car that was similar to what was worn by a person spotted in surveillance video taken the night of the first shooting.

Donaldson told investigators he was unfamiliar with the neighbourhood where the shootings occurred. He then asked for an attorney, but arrest records don't list one. The chief said he didn't know if Donaldson had a lawyer yet.

Donaldson graduated from St. Johns University in New York in January 2017, according to school 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.

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

The Tampa police chief said authorities have not been able to determine why Donaldson chose the Seminole Heights neighborhood.

Residents and police had been on edge since 9 October, when 22-year-old Benjamin Mitchell was shot to death. Authorities said Donaldson legally purchased the gun at a gun shop a few days before the killing.

On 11 October, 32-year-old Monica Hoffa was slain. And on 19 October 19, Anthony Naiboa, 20, was killed after taking the wrong bus home from his new job. On 14 November, 60-year-old Ronald Felton was killed.

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.

Dugan said the department had received more than 5,000 tips. He thanked those who called in the tip that led to Donaldson's arrest.