PHILADELPHIA — A passenger shot aboard a bus in Philadelphia has died from his wounds, marking the third time in three days that someone was killed while riding, entering or leaving a Southeastern Pennsylvania Transportation Authority (SEPTA) bus.

The most recent shooting occurred around 6:35 p.m. Tuesday, when police said a verbal argument and then a physical fight began on a bus. One of the two got off the vehicle, turned fired two shots from a 9 mm handgun, hitting a man later identified as 37-year-old Carmelo Drayton, who died shortly afterward at a hospital.

The shooter, who officials said was wearing one of the kinds of masks not allowed on the transit system, fled. Authorities are investigating possible motive, and no other injuries were reported.

SEPTA’s chief of transit police, Charles Lawson, said shots were fired at the victim while the driver was “immediately behind.”

The shooting came a day after a 17-year-old student was killed and four other people were wounded when gunfire erupted at a bus stop. The victims included two women who were riding on a bus.

Another fatal shooting occurred around 11:30 p.m. Sunday, when a 27-year-old man was killed by another passenger moments after they both got off a bus. Witnesses said the two had argued, but a motive remains under investigation.

No arrests have been made in any of the shootings, said Frank Vanore, deputy commissioner of the Philadelphia police department.

While serious crime overall is down along the transportation system, Lawson said, a pattern that has emerged over the past year and a half is people carrying weapons, usually illegally, getting into an argument and then opening fire. As a result, he vowed that officials would enforce crime aggressively and unapologetically, and use “every legal means at our disposal to target illegal gun possession.”

“We’re going to target individuals concealing their identity. We’re going to target fare evasion. We’re going to target open drug use,” Lawson said. “We’re going to target every criminal code on the books, and when the law provides for our officers the ability to protect themselves through frisks, through learning if individuals are armed, we’re going to do that, in every case.”

Lawson added that the number of people monitoring the system via cameras was being increased. Officials were also looking into ways to allow employees to report potential problems discreetly so as not to endanger their safety, and riders are encouraged to do the same.

“I understand the fear. ... What I hope they get is the fact that we’re committed,” Lawson said. “I have over 230 sworn officers absolutely committed to this fight, and they’re as frustrated and as angry as I am — and they’re engaged.”